diff --git a/arai60/21-29_Tree_BT_BST/21_104_Maximum Depth of Binary Tree/level_1.py b/arai60/21-29_Tree_BT_BST/21_104_Maximum Depth of Binary Tree/level_1.py
new file mode 100644
index 0000000..b20fee1
--- /dev/null
+++ b/arai60/21-29_Tree_BT_BST/21_104_Maximum Depth of Binary Tree/level_1.py	
@@ -0,0 +1,47 @@
+# DFS
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+
+        def search_max_depth(node):
+            if not node:
+                return 0
+            left_depth = search_max_depth(node.left) + 1
+            right_depth = search_max_depth(node.right) + 1
+            max_depth = max(left_depth, right_depth)
+            return max_depth
+
+        return search_max_depth(root)
+
+
+# BFS
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        node_depth_queue = deque([(root, 1)])
+        max_depth = 0
+        while node_depth_queue:
+            node, depth = node_depth_queue.popleft()
+            if node.left:
+                node_depth_queue.append((node.left, depth + 1))
+            if node.right:
+                node_depth_queue.append((node.right, depth + 1))
+            max_depth = max(max_depth, depth)
+        return max_depth
+
+
+# stackで深さ優先探索
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        stack_node_depth = [(root, 1)]
+        max_depth = 1
+        while stack_node_depth:
+            node, depth = stack_node_depth.pop()
+            if node.left:
+                stack_node_depth.append((node.left, depth + 1))
+            if node.right:
+                stack_node_depth.append((node.right, depth + 1))
+            max_depth = max(max_depth, depth)
+        return max_depth
diff --git a/arai60/21-29_Tree_BT_BST/21_104_Maximum Depth of Binary Tree/level_2.py b/arai60/21-29_Tree_BT_BST/21_104_Maximum Depth of Binary Tree/level_2.py
new file mode 100644
index 0000000..d26b371
--- /dev/null
+++ b/arai60/21-29_Tree_BT_BST/21_104_Maximum Depth of Binary Tree/level_2.py	
@@ -0,0 +1,45 @@
+# DFS
+# 特に変更なし
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        left_depth = self.maxDepth(root.left) + 1
+        right_depth = self.maxDepth(root.right) + 1
+        max_depth = max(left_depth, right_depth)
+        return max_depth
+
+
+# BFS
+# 特に変更なし
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        node_depth_queue = deque([(root, 1)])
+        while node_depth_queue:
+            node, depth = node_depth_queue.popleft()
+            if node.left:
+                node_depth_queue.append((node.left, depth + 1))
+            if node.right:
+                node_depth_queue.append((node.right, depth + 1))
+        max_depth = depth
+        return max_depth
+
+
+# stackで深さ優先探索
+# 特に変更なし
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        stack_node_depth = [(root, 1)]
+        max_depth = 1
+        while stack_node_depth:
+            node, depth = stack_node_depth.pop()
+            if node.left:
+                stack_node_depth.append((node.left, depth + 1))
+            if node.right:
+                stack_node_depth.append((node.right, depth + 1))
+            max_depth = max(max_depth, depth)
+        return max_depth
diff --git a/arai60/21-29_Tree_BT_BST/21_104_Maximum Depth of Binary Tree/level_3.py b/arai60/21-29_Tree_BT_BST/21_104_Maximum Depth of Binary Tree/level_3.py
new file mode 100644
index 0000000..8d321ea
--- /dev/null
+++ b/arai60/21-29_Tree_BT_BST/21_104_Maximum Depth of Binary Tree/level_3.py	
@@ -0,0 +1,12 @@
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+
+        def search_max_depth(node):
+            if not node:
+                return 0
+            left_depth = search_max_depth(node.left) + 1
+            right_depth = search_max_depth(node.right) + 1
+            max_depth = max(left_depth, right_depth)
+            return max_depth
+
+        return search_max_depth(root)