diff --git a/src/sing/stateMachine/sequencerStateMachine.ts b/src/sing/stateMachine/sequencerStateMachine.ts
new file mode 100644
index 0000000000..7091f1bf3d
--- /dev/null
+++ b/src/sing/stateMachine/sequencerStateMachine.ts
@@ -0,0 +1,269 @@
+/**
+ * このファイルのコードは実装中で、現在使われていません。
+ * issue: https://github.com/VOICEVOX/voicevox/issues/2041
+ */
+
+import { computed, ComputedRef, ref, Ref } from "vue";
+import { IState, StateMachine } from "@/sing/stateMachine/stateMachineBase";
+import {
+  getButton,
+  getDoremiFromNoteNumber,
+  isSelfEventTarget,
+  PREVIEW_SOUND_DURATION,
+} from "@/sing/viewHelper";
+import { Note, SequencerEditTarget } from "@/store/type";
+import { NoteId } from "@/type/preload";
+
+export type PositionOnSequencer = {
+  readonly ticks: number;
+  readonly noteNumber: number;
+};
+
+type Input =
+  | {
+      readonly targetArea: "SequencerBody";
+      readonly mouseEvent: MouseEvent;
+      readonly cursorPos: PositionOnSequencer;
+    }
+  | {
+      readonly targetArea: "Note";
+      readonly mouseEvent: MouseEvent;
+      readonly cursorPos: PositionOnSequencer;
+      readonly note: Note;
+    }
+  | {
+      readonly targetArea: "NoteLeftEdge";
+      readonly mouseEvent: MouseEvent;
+      readonly cursorPos: PositionOnSequencer;
+      readonly note: Note;
+    }
+  | {
+      readonly targetArea: "NoteRightEdge";
+      readonly mouseEvent: MouseEvent;
+      readonly cursorPos: PositionOnSequencer;
+      readonly note: Note;
+    };
+
+type ComputedRefs = {
+  readonly snapTicks: ComputedRef<number>;
+  readonly editTarget: ComputedRef<SequencerEditTarget>;
+};
+
+type Refs = {
+  readonly nowPreviewing: Ref<boolean>;
+  readonly previewNotes: Ref<Note[]>;
+  readonly guideLineTicks: Ref<number>;
+};
+
+type StoreActions = {
+  readonly deselectAllNotes: () => void;
+  readonly commandAddNotes: (notes: Note[]) => void;
+  readonly selectNotes: (noteIds: NoteId[]) => void;
+  readonly playPreviewSound: (noteNumber: number, duration?: number) => void;
+};
+
+type Context = ComputedRefs & Refs & { readonly storeActions: StoreActions };
+
+type State = IdleState | AddNoteState;
+
+const getGuideLineTicks = (
+  cursorPos: PositionOnSequencer,
+  context: Context,
+) => {
+  const cursorTicks = cursorPos.ticks;
+  const snapTicks = context.snapTicks.value;
+  // NOTE: 入力を補助する線の判定の境目はスナップ幅の3/4の位置
+  return Math.round(cursorTicks / snapTicks - 0.25) * snapTicks;
+};
+
+class IdleState implements IState<State, Input, Context> {
+  readonly id = "idle";
+
+  onEnter() {}
+
+  process({
+    input,
+    context,
+    setNextState,
+  }: {
+    input: Input;
+    context: Context;
+    setNextState: (nextState: State) => void;
+  }) {
+    const mouseButton = getButton(input.mouseEvent);
+    if (input.targetArea === "SequencerBody") {
+      context.guideLineTicks.value = getGuideLineTicks(
+        input.cursorPos,
+        context,
+      );
+      if (input.mouseEvent.type === "mousedown") {
+        // TODO: メニューが表示されている場合はメニュー非表示のみ行いたい
+        if (context.editTarget.value === "NOTE") {
+          if (!isSelfEventTarget(input.mouseEvent)) {
+            return;
+          }
+          if (mouseButton === "LEFT_BUTTON") {
+            if (
+              input.cursorPos.ticks < 0 ||
+              input.cursorPos.noteNumber < 0 ||
+              input.cursorPos.noteNumber > 127
+            ) {
+              return;
+            }
+            setNextState(new AddNoteState(input.cursorPos));
+          }
+        }
+      }
+    }
+  }
+
+  onExit() {}
+}
+
+class AddNoteState implements IState<State, Input, Context> {
+  readonly id = "addNote";
+
+  private readonly cursorPosAtStart: PositionOnSequencer;
+
+  private currentCursorPos: PositionOnSequencer;
+  private innerContext:
+    | {
+        noteToAdd: Note;
+        previewRequestId: number;
+        executePreviewProcess: boolean;
+      }
+    | undefined;
+
+  constructor(cursorPosAtStart: PositionOnSequencer) {
+    this.cursorPosAtStart = cursorPosAtStart;
+    this.currentCursorPos = cursorPosAtStart;
+  }
+
+  private previewAdd(context: Context) {
+    if (this.innerContext == undefined) {
+      throw new Error("innerContext is undefined.");
+    }
+    const noteToAdd = this.innerContext.noteToAdd;
+    const snapTicks = context.snapTicks.value;
+    const dragTicks = this.currentCursorPos.ticks - this.cursorPosAtStart.ticks;
+    const noteDuration = Math.round(dragTicks / snapTicks) * snapTicks;
+    const noteEndPos = noteToAdd.position + noteDuration;
+    const previewNotes = context.previewNotes.value;
+
+    const editedNotes = new Map<NoteId, Note>();
+    for (const note of previewNotes) {
+      const duration = Math.max(snapTicks, noteDuration);
+      if (note.duration !== duration) {
+        editedNotes.set(note.id, { ...note, duration });
+      }
+    }
+    if (editedNotes.size !== 0) {
+      context.previewNotes.value = previewNotes.map((value) => {
+        return editedNotes.get(value.id) ?? value;
+      });
+    }
+    context.guideLineTicks.value = noteEndPos;
+  }
+
+  onEnter(context: Context) {
+    context.storeActions.deselectAllNotes();
+
+    const guideLineTicks = getGuideLineTicks(this.cursorPosAtStart, context);
+    const noteToAdd = {
+      id: NoteId(crypto.randomUUID()),
+      position: guideLineTicks,
+      duration: context.snapTicks.value,
+      noteNumber: this.cursorPosAtStart.noteNumber,
+      lyric: getDoremiFromNoteNumber(this.cursorPosAtStart.noteNumber),
+    };
+
+    context.previewNotes.value = [noteToAdd];
+    context.nowPreviewing.value = true;
+
+    const preview = () => {
+      if (this.innerContext == undefined) {
+        throw new Error("innerContext is undefined.");
+      }
+      if (this.innerContext.executePreviewProcess) {
+        this.previewAdd(context);
+        this.innerContext.executePreviewProcess = false;
+      }
+      this.innerContext.previewRequestId = requestAnimationFrame(preview);
+    };
+    const previewRequestId = requestAnimationFrame(preview);
+
+    this.innerContext = {
+      noteToAdd,
+      executePreviewProcess: false,
+      previewRequestId,
+    };
+  }
+
+  process({
+    input,
+    setNextState,
+  }: {
+    input: Input;
+    context: Context;
+    setNextState: (nextState: State) => void;
+  }) {
+    if (this.innerContext == undefined) {
+      throw new Error("innerContext is undefined.");
+    }
+    const mouseButton = getButton(input.mouseEvent);
+    if (input.targetArea === "SequencerBody") {
+      if (input.mouseEvent.type === "mousemove") {
+        this.currentCursorPos = input.cursorPos;
+        this.innerContext.executePreviewProcess = true;
+      } else if (input.mouseEvent.type === "mouseup") {
+        if (mouseButton === "LEFT_BUTTON") {
+          setNextState(new IdleState());
+        }
+      }
+    }
+  }
+
+  onExit(context: Context) {
+    if (this.innerContext == undefined) {
+      throw new Error("innerContext is undefined.");
+    }
+    const previewNotes = context.previewNotes.value;
+    const previewNoteIds = previewNotes.map((value) => value.id);
+
+    cancelAnimationFrame(this.innerContext.previewRequestId);
+    context.storeActions.commandAddNotes(context.previewNotes.value);
+    context.storeActions.selectNotes(previewNoteIds);
+    if (previewNotes.length === 1) {
+      context.storeActions.playPreviewSound(
+        previewNotes[0].noteNumber,
+        PREVIEW_SOUND_DURATION,
+      );
+    }
+    context.nowPreviewing.value = false;
+  }
+}
+
+export const useSequencerStateMachine = (
+  computedRefs: ComputedRefs,
+  storeActions: StoreActions,
+) => {
+  const refs: Refs = {
+    nowPreviewing: ref(false),
+    previewNotes: ref<Note[]>([]),
+    guideLineTicks: ref(0),
+  };
+  const stateMachine = new StateMachine<State, Input, Context>(
+    new IdleState(),
+    {
+      ...computedRefs,
+      ...refs,
+      storeActions,
+    },
+  );
+  return {
+    stateMachine,
+    nowPreviewing: computed(() => refs.nowPreviewing.value),
+    previewNotes: computed(() => refs.previewNotes.value),
+    guideLineTicks: computed(() => refs.guideLineTicks.value),
+  };
+};
diff --git a/src/sing/stateMachine/stateMachineBase.ts b/src/sing/stateMachine/stateMachineBase.ts
new file mode 100644
index 0000000000..6d6c5a09ac
--- /dev/null
+++ b/src/sing/stateMachine/stateMachineBase.ts
@@ -0,0 +1,101 @@
+/**
+ * このファイルのコードは実装中で、現在使われていません。
+ * issue: https://github.com/VOICEVOX/voicevox/issues/2041
+ */
+
+/**
+ * ステートマシンのステートを表すインターフェース。
+ *
+ * @template State このインターフェースを実装するステートの型。
+ * @template Input ステートが処理する入力の型。
+ * @template Context ステート間で共有されるコンテキストの型。
+ */
+export interface IState<
+  State extends IState<State, Input, Context>,
+  Input,
+  Context,
+> {
+  /**
+   * 入力を処理し、必要に応じて次のステートを設定する。
+   *
+   * @param payload `input`、`context`、`setNextState`関数を含むペイロード。
+   */
+  process(payload: {
+    input: Input;
+    context: Context;
+    setNextState: (nextState: State) => void;
+  }): void;
+
+  /**
+   * ステートに入ったときに呼び出される。
+   *
+   * @param context ステート間で共有されるコンテキスト。
+   */
+  onEnter(context: Context): void;
+
+  /**
+   * ステートから出るときに呼び出される。
+   *
+   * @param context ステート間で共有されるコンテキスト。
+   */
+  onExit(context: Context): void;
+}
+
+/**
+ * ステートマシンを表すクラス。
+ *
+ * @template State ステートマシンのステートの型。
+ * @template Input ステートが処理する入力の型。
+ * @template Context ステート間で共有されるコンテキストの型。
+ */
+export class StateMachine<
+  State extends IState<State, Input, Context>,
+  Input,
+  Context,
+> {
+  private readonly context: Context;
+
+  private currentState: State;
+
+  /**
+   * @param initialState ステートマシンの初期ステート。
+   * @param context ステート間で共有されるコンテキスト。
+   */
+  constructor(initialState: State, context: Context) {
+    this.context = context;
+    this.currentState = initialState;
+
+    this.currentState.onEnter(this.context);
+  }
+
+  /**
+   * ステートマシンの現在のステートを返す。
+   *
+   * @returns 現在のステート。
+   */
+  getCurrentState() {
+    return this.currentState;
+  }
+
+  /**
+   * 現在のステートを使用して入力を処理し、必要に応じてステートの遷移を行う。
+   *
+   * @param input 処理する入力。
+   */
+  process(input: Input) {
+    let nextState: State | undefined = undefined;
+    const setNextState = (arg: State) => {
+      nextState = arg;
+    };
+    this.currentState.process({
+      input,
+      context: this.context,
+      setNextState,
+    });
+    if (nextState != undefined) {
+      this.currentState.onExit(this.context);
+      this.currentState = nextState;
+      this.currentState.onEnter(this.context);
+    }
+  }
+}
diff --git a/src/sing/viewHelper.ts b/src/sing/viewHelper.ts
index b20366eaa9..0665a0bd12 100644
--- a/src/sing/viewHelper.ts
+++ b/src/sing/viewHelper.ts
@@ -149,3 +149,16 @@ export function getButton(event: MouseEvent): MouseButton {
     return "OTHER_BUTTON";
   }
 }
+
+export const getXInBorderBox = (clientX: number, element: HTMLElement) => {
+  return clientX - element.getBoundingClientRect().left;
+};
+
+export const getYInBorderBox = (clientY: number, element: HTMLElement) => {
+  return clientY - element.getBoundingClientRect().top;
+};
+
+/** 直接イベントが来ているかどうか */
+export const isSelfEventTarget = (event: UIEvent) => {
+  return event.target === event.currentTarget;
+};