diff --git a/include/picrin/box.h b/include/picrin/box.h
deleted file mode 100644
index f9826eed..00000000
--- a/include/picrin/box.h
+++ /dev/null
@@ -1,28 +0,0 @@
-/**
- * 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_value pic_box(pic_state *, pic_value);
-pic_value pic_unbox(pic_state *, pic_value);
-void pic_set_box(pic_state *, pic_value, pic_value);
-
-#if defined(__cplusplus)
-}
-#endif
-
-#endif
diff --git a/include/picrin/value.h b/include/picrin/value.h
index e8eb7342..283bac28 100644
--- a/include/picrin/value.h
+++ b/include/picrin/value.h
@@ -115,7 +115,6 @@ enum pic_tt {
   PIC_TT_VAR,
   PIC_TT_IREP,
   PIC_TT_DATA,
-  PIC_TT_BOX,
   PIC_TT_DICT
 };
 
@@ -267,8 +266,6 @@ pic_type_repr(enum pic_tt tt)
     return "irep";
   case PIC_TT_DATA:
     return "data";
-  case PIC_TT_BOX:
-    return "box";
   case PIC_TT_DICT:
     return "dict";
   }
diff --git a/src/box.c b/src/box.c
deleted file mode 100644
index b9948fc7..00000000
--- a/src/box.c
+++ /dev/null
@@ -1,30 +0,0 @@
-#include "picrin.h"
-#include "picrin/box.h"
-
-pic_value
-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 pic_obj_value(box);
-}
-
-pic_value
-pic_unbox(pic_state *pic, pic_value box)
-{
-  if (! pic_box_p(box)) {
-    pic_errorf(pic, "expected box, but got ~s", box);
-  }
-  return pic_box_ptr(box)->value;
-}
-
-void
-pic_set_box(pic_state *pic, pic_value box, pic_value value)
-{
-  if (! pic_box_p(box)) {
-    pic_errorf(pic, "expected box, but got ~s", box);
-  }
-  pic_box_ptr(box)->value = value;
-}
diff --git a/src/codegen.c b/src/codegen.c
index d098842e..df4c0239 100644
--- a/src/codegen.c
+++ b/src/codegen.c
@@ -830,7 +830,6 @@ analyze_node(analyze_state *state, pic_value obj, bool tailpos)
   case PIC_TT_VAR:
   case PIC_TT_IREP:
   case PIC_TT_DATA:
-  case PIC_TT_BOX:
   case PIC_TT_DICT:
     pic_errorf(pic, "invalid expression given: ~s", obj);
   }
diff --git a/src/gc.c b/src/gc.c
index 21aebb9e..aa2d383d 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -494,11 +494,6 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
     }
     break;
   }
-  case PIC_TT_BOX: {
-    struct pic_box *box = (struct pic_box *)obj;
-    gc_mark(pic, box->value);
-    break;
-  }
   case PIC_TT_DICT: {
     struct pic_dict *dict = (struct pic_dict *)obj;
     xh_iter it;
@@ -656,9 +651,6 @@ gc_finalize_object(pic_state *pic, struct pic_object *obj)
     xh_destroy(&data->storage);
     break;
   }
-  case PIC_TT_BOX: {
-    break;
-  }
   case PIC_TT_DICT: {
     struct pic_dict *dict = (struct pic_dict *)obj;
     xh_destroy(&dict->hash);
diff --git a/src/macro.c b/src/macro.c
index 49fbe345..636a968e 100644
--- a/src/macro.c
+++ b/src/macro.c
@@ -524,7 +524,6 @@ macroexpand_node(pic_state *pic, pic_value expr, struct pic_senv *senv)
   case PIC_TT_VAR:
   case PIC_TT_IREP:
   case PIC_TT_DATA:
-  case PIC_TT_BOX:
   case PIC_TT_DICT:
     pic_errorf(pic, "unexpected value type: ~s", expr);
   }
diff --git a/src/write.c b/src/write.c
index 9ced3904..61551b1a 100644
--- a/src/write.c
+++ b/src/write.c
@@ -330,9 +330,6 @@ write_core(struct writer_control *p, pic_value obj)
   case PIC_TT_DATA:
     xfprintf(file, "#<data %p>", pic_ptr(obj));
     break;
-  case PIC_TT_BOX:
-    xfprintf(file, "#<box %p>", pic_ptr(obj));
-    break;
   case PIC_TT_DICT:
     xfprintf(file, "#<dict %p>", pic_ptr(obj));
     break;