diff --git a/line/handler/index.js b/line/handler/index.js
index 817f2d4..6759870 100644
--- a/line/handler/index.js
+++ b/line/handler/index.js
@@ -8,6 +8,7 @@ const lineEventHander = middlewareCompose([
   require('./cmd'), // 處理指令
   require('./replyFlexFromText'), // 嘗試回傳 flex
   require('./noReplyUrl'), // 如果是一個合法的 URL 就不作回應
+  require('./replyCalc'), // 計算數學式
   require('./replyEventJson'), // 把事件用 json 回傳
 ])
 
diff --git a/line/handler/replyCalc.js b/line/handler/replyCalc.js
new file mode 100644
index 0000000..5d9e2e4
--- /dev/null
+++ b/line/handler/replyCalc.js
@@ -0,0 +1,13 @@
+const _ = require('lodash')
+
+module.exports = async (ctx, next) => {
+  const text = ctx?.event?.message?.text
+  try {
+    if (!_.isString(text)) throw new Error('Not a string')
+    if (/[^0-9!.()*/&%^+<>|~eE -]/.test(text)) throw new Error('Not a arithmetic string')
+    return new Function(`return ${text}`)().toString() // eslint-disable-line no-new-func
+  } catch (err) {
+    // console.log(err)
+    return await next()
+  }
+}