diff --git a/include/picrin/string.h b/include/picrin/string.h
deleted file mode 100644
index fa64b6de..00000000
--- a/include/picrin/string.h
+++ /dev/null
@@ -1,12 +0,0 @@
-#ifndef STRING_H__
-#define STRING_H__
-
-struct pic_string {
-  PIC_OBJECT_HEADER
-  char *str;
-  size_t len;
-};
-
-#define pic_str_ptr(v) ((struct pic_string *)v.u.data)
-
-#endif
diff --git a/include/picrin/value.h b/include/picrin/value.h
index 212466e0..2dcf3673 100644
--- a/include/picrin/value.h
+++ b/include/picrin/value.h
@@ -50,12 +50,19 @@ struct pic_symbol {
   char *name;
 };
 
+struct pic_string {
+  PIC_OBJECT_HEADER
+  char *str;
+  size_t len;
+};
+
 struct pic_proc;
 struct pic_port;
 
 #define pic_obj_ptr(o) ((struct pic_object *)o.u.data)
 #define pic_pair_ptr(o) ((struct pic_pair *)o.u.data)
 #define pic_symbol_ptr(o) ((struct pic_symbol *)o.u.data)
+#define pic_str_ptr(v) ((struct pic_string *)v.u.data)
 
 enum pic_tt pic_type(pic_value);
 
diff --git a/src/gc.c b/src/gc.c
index c977a91c..6a59179a 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -4,7 +4,6 @@
 #include "picrin/gc.h"
 #include "picrin/irep.h"
 #include "picrin/proc.h"
-#include "picrin/string.h"
 #include "picrin/symbol.h"
 
 #if GC_DEBUG
diff --git a/src/port.c b/src/port.c
index d6de83d6..e41edbf0 100644
--- a/src/port.c
+++ b/src/port.c
@@ -4,7 +4,6 @@
 #include "picrin.h"
 #include "picrin/proc.h"
 #include "picrin/port.h"
-#include "picrin/string.h"
 
 static void write_pair(pic_state *pic, struct pic_pair *pair);
 static void write_str(pic_state *pic, struct pic_string *str);
diff --git a/src/string.c b/src/string.c
index 896ebf12..09806b25 100644
--- a/src/string.c
+++ b/src/string.c
@@ -1,7 +1,6 @@
 #include <string.h>
 
 #include "picrin.h"
-#include "picrin/string.h"
 
 pic_value
 pic_str_new(pic_state *pic, const char *cstr, size_t len)
diff --git a/src/vm.c b/src/vm.c
index 30d64f79..046c3624 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -5,7 +5,6 @@
 #include "picrin/pair.h"
 #include "picrin/proc.h"
 #include "picrin/irep.h"
-#include "picrin/string.h"
 
 #define GET_OPERAND(pic,n) ((pic)->sp[-1-(n)])