diff --git a/.Lib9c.Tests/Model/State/CollectionStateTest.cs b/.Lib9c.Tests/Model/State/CollectionStateTest.cs
index bdd8d6d862..d4519be9c6 100644
--- a/.Lib9c.Tests/Model/State/CollectionStateTest.cs
+++ b/.Lib9c.Tests/Model/State/CollectionStateTest.cs
@@ -2,8 +2,6 @@ namespace Lib9c.Tests.Model.State
 {
     using System.Collections.Generic;
     using Bencodex.Types;
-    using Libplanet.Crypto;
-    using Nekoyume.Action;
     using Nekoyume.Model.State;
     using Xunit;
 
@@ -14,7 +12,7 @@ public void Bencoded()
         {
             var state = new CollectionState
             {
-                Ids = new List<int>
+                Ids = new SortedSet<int>
                 {
                     1,
                     2,
diff --git a/.Lib9c.Tests/Module/CollectionModuleTest.cs b/.Lib9c.Tests/Module/CollectionModuleTest.cs
index a996886239..fe3e7c5cc5 100644
--- a/.Lib9c.Tests/Module/CollectionModuleTest.cs
+++ b/.Lib9c.Tests/Module/CollectionModuleTest.cs
@@ -22,7 +22,7 @@ public void CollectionState()
 
             var state = new CollectionState
             {
-                Ids = new List<int>
+                Ids = new SortedSet<int>
                 {
                     1,
                 },
@@ -47,7 +47,7 @@ public void CollectionStates()
 
             var state = new CollectionState
             {
-                Ids = new List<int>
+                Ids = new SortedSet<int>
                 {
                     1,
                 },
diff --git a/Lib9c/Model/State/CollectionState.cs b/Lib9c/Model/State/CollectionState.cs
index 6966218390..1458e83bc4 100644
--- a/Lib9c/Model/State/CollectionState.cs
+++ b/Lib9c/Model/State/CollectionState.cs
@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using System.Linq;
 using Bencodex;
 using Bencodex.Types;
 using Nekoyume.Model.Stat;
@@ -12,7 +11,7 @@ namespace Nekoyume.Model.State
     /// </summary>
     public class CollectionState : IBencodable
     {
-        public List<int> Ids = new();
+        public SortedSet<int> Ids = new();
 
         public CollectionState()
         {
@@ -25,15 +24,13 @@ public CollectionState(List serialized)
             {
                 Ids.Add((Integer)value);
             }
-
-            Ids = Ids.Distinct().ToList();
         }
 
         public CollectionState(IValue bencoded) : this((List)bencoded)
         {
         }
 
-        public IValue Bencoded => List.Empty.Add(new List(Ids.Distinct()));
+        public IValue Bencoded => List.Empty.Add(new List(Ids));
 
         public List<StatModifier> GetModifiers(CollectionSheet collectionSheet)
         {