diff --git a/ChangeLog b/ChangeLog
index b36e6958a2..cb15973eb7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,17 @@
+Sun May 15 03:13:01 2016  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* iseq.h (struct iseq_compile_data): use struct rb_id_table
+	  instead of st_table.
+
+	* iseq.c (prepare_iseq_build): don't allocate ivar_cache_table
+	  until it has at least one element.
+
+	* iseq.c (compile_data_free): free ivar_cache_table only if it
+	  is allocated.
+
+	* compile.c (get_ivar_ic_value): allocate if the table is not
+	  allocated yet.
+
 Sat May 14 09:04:34 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/mkmf.rb (pkg_config): use xsystem consistently to set up
diff --git a/compile.c b/compile.c
index 8dc2abdfcf..cc496cbab8 100644
--- a/compile.c
+++ b/compile.c
@@ -19,6 +19,7 @@
 #include "iseq.h"
 #include "insns.inc"
 #include "insns_info.inc"
+#include "id_table.h"
 #include "gc.h"
 
 #ifdef HAVE_DLADDR
@@ -1547,11 +1548,18 @@ static inline VALUE
 get_ivar_ic_value(rb_iseq_t *iseq,ID id)
 {
     VALUE val;
-    st_table *tbl = ISEQ_COMPILE_DATA(iseq)->ivar_cache_table;
-    if(!st_lookup(tbl,(st_data_t)id,&val)){
-	val = INT2FIX(iseq->body->is_size++);
-	st_insert(tbl,id,val);
+    struct rb_id_table *tbl = ISEQ_COMPILE_DATA(iseq)->ivar_cache_table;
+    if (tbl) {
+	if (rb_id_table_lookup(tbl,id,&val)) {
+	    return val;
+	}
     }
+    else {
+	tbl = rb_id_table_create(1);
+	ISEQ_COMPILE_DATA(iseq)->ivar_cache_table = tbl;
+    }
+    val = INT2FIX(iseq->body->is_size++);
+    rb_id_table_insert(tbl,id,val);
     return val;
 }
 
diff --git a/iseq.c b/iseq.c
index 2a4d58e611..28d35eb328 100644
--- a/iseq.c
+++ b/iseq.c
@@ -21,6 +21,7 @@
 #include "gc.h"
 #include "vm_core.h"
 #include "iseq.h"
+#include "id_table.h"
 
 #include "insns.inc"
 #include "insns_info.inc"
@@ -58,8 +59,9 @@ compile_data_free(struct iseq_compile_data *compile_data)
 	    ruby_xfree(cur);
 	    cur = next;
 	}
-	st_free_table(compile_data->ivar_cache_table);
-
+	if (compile_data->ivar_cache_table) {
+	    rb_id_table_free(compile_data->ivar_cache_table);
+	}
 	ruby_xfree(compile_data);
     }
 }
@@ -300,7 +302,7 @@ prepare_iseq_build(rb_iseq_t *iseq,
     ISEQ_COMPILE_DATA(iseq)->option = option;
     ISEQ_COMPILE_DATA(iseq)->last_coverable_line = -1;
 
-    ISEQ_COMPILE_DATA(iseq)->ivar_cache_table = st_init_numtable();
+    ISEQ_COMPILE_DATA(iseq)->ivar_cache_table = NULL;
 
     if (option->coverage_enabled) {
 	VALUE coverages = rb_get_coverages();
diff --git a/iseq.h b/iseq.h
index d6b0e54820..6a5cb8bdad 100644
--- a/iseq.h
+++ b/iseq.h
@@ -213,7 +213,7 @@ struct iseq_compile_data {
     unsigned int ci_index;
     unsigned int ci_kw_index;
     const rb_compile_option_t *option;
-    st_table *ivar_cache_table;
+    struct rb_id_table *ivar_cache_table;
 #if SUPPORT_JOKE
     st_table *labels_table;
 #endif