diff --git a/extlib/benz/gc.c b/extlib/benz/gc.c
index 2e7ae20f..a7096cca 100644
--- a/extlib/benz/gc.c
+++ b/extlib/benz/gc.c
@@ -35,6 +35,7 @@ union object {
   struct pic_port port;
   struct pic_error err;
   struct pic_lib lib;
+  struct pic_box box;
   struct pic_checkpoint cp;
 };
 
@@ -405,6 +406,12 @@ gc_mark_object(pic_state *pic, union object *obj)
     pic->heap->regs = reg;
     break;
   }
+  case PIC_TT_BOX: {
+    if (pic_obj_p(obj->box.value)) {
+      LOOP(pic_obj_ptr(obj->box.value));
+    }
+    break;
+  }
   case PIC_TT_CP: {
     if (obj->cp.prev) {
       gc_mark_object(pic, (union object *)obj->cp.prev);
@@ -597,6 +604,7 @@ gc_finalize_object(pic_state *pic, union object *obj)
   case PIC_TT_LIB:
   case PIC_TT_RECORD:
   case PIC_TT_CP:
+  case PIC_TT_BOX:
     break;
 
   case PIC_TT_NIL:
diff --git a/extlib/benz/include/picrin.h b/extlib/benz/include/picrin.h
index 30ec8a58..da2a4885 100644
--- a/extlib/benz/include/picrin.h
+++ b/extlib/benz/include/picrin.h
@@ -268,6 +268,7 @@ pic_value pic_fdisplay(pic_state *, pic_value, xFILE *);
 #include "picrin/symbol.h"
 #include "picrin/vector.h"
 #include "picrin/reg.h"
+#include "picrin/box.h"
 
 #if defined(__cplusplus)
 }
diff --git a/extlib/benz/include/picrin/box.h b/extlib/benz/include/picrin/box.h
new file mode 100644
index 00000000..dcfa676a
--- /dev/null
+++ b/extlib/benz/include/picrin/box.h
@@ -0,0 +1,34 @@
+/**
+ * See Copyright Notice in picrin.h
+ */
+
+#ifndef PICRIN_BOX_H
+#define PICRIN_BOX_H
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+struct pic_box {
+  PIC_OBJECT_HEADER
+  pic_value value;
+};
+
+#define pic_box_p(v) (pic_type(v) == PIC_TT_BOX)
+#define pic_box_ptr(v) ((struct pic_box *)pic_ptr(v))
+
+PIC_INLINE struct pic_box *
+pic_box(pic_state *pic, pic_value value)
+{
+  struct pic_box *box;
+
+  box = (struct pic_box *)pic_obj_alloc(pic, sizeof(struct pic_box), PIC_TT_BOX);
+  box->value = value;
+  return box;
+}
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif
diff --git a/extlib/benz/include/picrin/type.h b/extlib/benz/include/picrin/type.h
index 0df475a8..4de8d279 100644
--- a/extlib/benz/include/picrin/type.h
+++ b/extlib/benz/include/picrin/type.h
@@ -156,6 +156,7 @@ enum pic_tt {
   PIC_TT_DICT,
   PIC_TT_REG,
   PIC_TT_RECORD,
+  PIC_TT_BOX,
   PIC_TT_CXT,
   PIC_TT_IREP,
   PIC_TT_CP
@@ -313,6 +314,8 @@ pic_type_repr(enum pic_tt tt)
     return "dict";
   case PIC_TT_REG:
     return "reg";
+  case PIC_TT_BOX:
+    return "box";
   case PIC_TT_RECORD:
     return "record";
   case PIC_TT_CP: