diff --git a/include/ruby/st.h b/include/ruby/st.h
index d23f761e7e..84ef72b440 100644
--- a/include/ruby/st.h
+++ b/include/ruby/st.h
@@ -76,7 +76,7 @@ st_table *st_init_strtable(void);
 st_table *st_init_strtable_with_size(int);
 st_table *st_init_strcasetable(void);
 st_table *st_init_strcasetable_with_size(int);
-int st_delete(st_table *, st_data_t *, st_data_t *);
+int st_delete(st_table *, st_data_t *, st_data_t *); /* returns 0:notfound 1:deleted */
 int st_delete_safe(st_table *, st_data_t *, st_data_t *, st_data_t);
 int st_insert(st_table *, st_data_t, st_data_t);
 int st_lookup(st_table *, st_data_t, st_data_t *);