diff --git a/id_table.h b/id_table.h
index 0b1b119b84..167ff47001 100644
--- a/id_table.h
+++ b/id_table.h
@@ -1,3 +1,5 @@
+#ifndef RUBY_ID_TABLE_H
+#define RUBY_ID_TABLE_H 1
 #include "ruby/ruby.h"
 
 struct rb_id_table;
@@ -24,3 +26,5 @@ typedef enum rb_id_table_iterator_result rb_id_table_foreach_func_t(ID id, VALUE
 typedef enum rb_id_table_iterator_result rb_id_table_foreach_values_func_t(VALUE val, void *data);
 void rb_id_table_foreach(struct rb_id_table *tbl, rb_id_table_foreach_func_t *func, void *data);
 void rb_id_table_foreach_values(struct rb_id_table *tbl, rb_id_table_foreach_values_func_t *func, void *data);
+
+#endif	/* RUBY_ID_TABLE_H */