diff --git a/package.json b/package.json
index d3d279e..35033c1 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "serializable-bptree",
-  "version": "4.0.0",
+  "version": "4.0.1",
   "description": "Store the B+tree flexibly, not only in-memory.",
   "main": "dist/cjs/index.cjs",
   "module": "dist/esm/index.mjs",
diff --git a/src/base/BPTree.ts b/src/base/BPTree.ts
index 0bec61d..c97af28 100644
--- a/src/base/BPTree.ts
+++ b/src/base/BPTree.ts
@@ -2,6 +2,7 @@ import { CacheBranchSync } from 'cachebranch'
 
 import { ValueComparator } from './ValueComparator'
 import { SerializableData, SerializeStrategy } from './SerializeStrategy'
+import { InvertedWeakMap } from '../utils/InvertedWeakMap'
 
 type Sync<T> = T
 type Async<T> = Promise<T>
@@ -55,7 +56,7 @@ export abstract class BPTree<K, V> {
 
   protected readonly strategy: SerializeStrategy<K, V>
   protected readonly comparator: ValueComparator<V>
-  protected readonly nodes: Map<string, BPTreeUnknownNode<K, V>>
+  protected readonly nodes: InvertedWeakMap<string, BPTreeUnknownNode<K, V>>
   protected order!: number
   protected root!: BPTreeUnknownNode<K, V>
 
@@ -122,7 +123,7 @@ export abstract class BPTree<K, V> {
     this._nodeCreateBuffer = new Map()
     this._nodeUpdateBuffer = new Map()
     this._nodeDeleteBuffer = new Map()
-    this.nodes = new Map()
+    this.nodes = new InvertedWeakMap()
     this.strategy = strategy
     this.comparator = comparator
   }
diff --git a/src/utils/InvertedWeakMap.ts b/src/utils/InvertedWeakMap.ts
new file mode 100644
index 0000000..64a015a
--- /dev/null
+++ b/src/utils/InvertedWeakMap.ts
@@ -0,0 +1,39 @@
+export class InvertedWeakMap<K extends string|number|symbol, V extends WeakKey> {
+  private readonly _map: Map<K, WeakRef<V>>
+  private readonly _registry: FinalizationRegistry<K>
+
+  constructor() {
+    this._map = new Map()
+    this._registry = new FinalizationRegistry((key) => this._map.delete(key))
+  }
+
+  clear(): void {
+    return this._map.clear()
+  }
+
+  delete(key: K): boolean {
+    return this._map.delete(key)
+  }
+
+  get(key: K): V|undefined {
+    return this._map.get(key)?.deref()
+  }
+
+  has(key: K): boolean {
+    return this._map.has(key) && this.get(key) !== undefined
+  }
+
+  set(key: K, value: V): this {
+    this._map.set(key, new WeakRef(value))
+    this._registry.register(value, key)
+    return this
+  }
+
+  get size(): number {
+    return this._map.size
+  }
+
+  keys(): IterableIterator<K> {
+    return this._map.keys()
+  }
+}