diff --git a/src/BPTree.c b/src/BPTree.c
index 42fc33a0dafeeb8e4c70daac02443c6d328da270..9c56ea24b85d978911e5b46c81470236310dbc61 100644
--- a/src/BPTree.c
+++ b/src/BPTree.c
@@ -110,6 +110,13 @@ bool BPTree_search(BPTreeNode *root, uint64_t key, uint64_t *data) {
 
 // "traverse" function for insertion and deletion.
 
+/**
+ * @brief Finds the next node from a node with respect to a given key.
+ *
+ * @param node The origin node.
+ * @param key The key that must be compared.
+ * @return BPTreeNode* The next node.
+ */
 static BPTreeNode *traverse(BPTreeNode *node, uint64_t key) {
     int virtual_insertion_index = IntegerArray_lower_bound(node->keys, key);