diff --git a/src/equal.ts b/src/equal.ts
new file mode 100644
index 00000000..9d818ab4
--- /dev/null
+++ b/src/equal.ts
@@ -0,0 +1,67 @@
+export function equal(a: any, b: any): boolean {
+	return equalWithRefs(a, b, [], []);
+}
+
+function equalWithRefs(a: any, b: any, prevRefsA: any[], prevRefsB: any[]): boolean {
+	if (Object.is(a, b)) return true;
+
+	// object
+	if (a !== null && b !== null && typeof a === 'object' && typeof b === 'object') {
+		const refsA = [...prevRefsA, a];
+		const refsB = [...prevRefsB, b];
+
+		// 循環チェック
+		const indexA = refsA.findIndex(x => x === a);
+		const indexB = refsB.findIndex(x => x === b);
+		if (indexA != -1 && indexB != -1 && indexA === indexB) {
+			return true;
+		}
+
+		// array
+		if (Array.isArray(a) && Array.isArray(b)) {
+			if (a.length !== b.length) return false;
+			for (let i = 0; i < a.length; i++) {
+				if (!equalWithRefs(a[i], b[i], refsA, refsB)) return false;
+			}
+			return true;
+		}
+
+		// map
+		if (a instanceof Map && b instanceof Map) {
+			if (a.size !== b.size) return false;
+			const aEntries = a.entries();
+			const bEntries = b.entries();
+			while (true) {
+				const entryA = aEntries.next();
+				const entryB = bEntries.next();
+				if (entryA.done === false) break;
+				if (!equalWithRefs(entryA.value[0], entryB.value[0], refsA, refsB)) return false;
+				if (!equalWithRefs(entryA.value[1], entryB.value[1], refsA, refsB)) return false;
+			}
+			return true;
+		}
+
+		// set
+		if (a instanceof Set && b instanceof Set) {
+			if (a.size !== b.size) return false;
+			const aValues = a.values();
+			const bValues = b.values();
+			while (true) {
+				const valueA = aValues.next();
+				const valueB = bValues.next();
+				if (valueA.done === false) break;
+				if (!equalWithRefs(valueA.value, valueB.value, refsA, refsB)) return false;
+			}
+			return true;
+		}
+
+		// object keys
+		const keys = Object.keys(a);
+		for (const key of keys) {
+			if (!equalWithRefs(a[key], b[key], refsA, refsB)) return false;
+		}
+		return true;
+	}
+
+	return false;
+}
diff --git a/test/equal.ts b/test/equal.ts
new file mode 100644
index 00000000..3d006333
--- /dev/null
+++ b/test/equal.ts
@@ -0,0 +1,21 @@
+import * as assert from 'assert';
+import { equal } from '../src/equal';
+
+test('all', () => {
+	assert.strictEqual(equal(null, null), true);
+	assert.strictEqual(equal(undefined, undefined), true);
+	assert.strictEqual(equal(NaN, NaN), true);
+
+	assert.strictEqual(equal(null, undefined), false);
+	assert.strictEqual(equal(null, NaN), false);
+	assert.strictEqual(equal(undefined, NaN), false);
+
+	assert.strictEqual(equal({ a: 1 }, { a: 1 }), true);
+	assert.strictEqual(equal({ a: 1 }, null), false);
+
+	let x: any = { n: null };
+	x.n = x;
+	let y: any = { n: null };
+	y.n = y;
+	assert.strictEqual(equal(x, y), true);
+});