diff --git a/extlib/benz/read.c b/extlib/benz/read.c
index dc7a28a5..a43b28de 100644
--- a/extlib/benz/read.c
+++ b/extlib/benz/read.c
@@ -560,6 +560,21 @@ read_blob(pic_state *pic, struct pic_port *port, int c)
   return pic_obj_value(blob);
 }
 
+static pic_value
+read_undef_or_blob(pic_state *pic, struct pic_port *port, int c)
+{
+  if ((c = peek(port)) == 'n') {
+    if (! expect(port, "ndefined")) {
+      read_error(pic, "unexpected character while reading #undefined");
+    }
+    return pic_undef_value();
+  }
+  if (! isdigit(c)) {
+    read_error(pic, "expect #undefined or #u8(...), but illegal character given");
+  }
+  return read_blob(pic, port, 'u');
+}
+
 static pic_value
 read_pair(pic_state *pic, struct pic_port *port, int c)
 {
@@ -786,7 +801,7 @@ reader_table_init(struct pic_reader *reader)
   reader->dispatch['f'] = read_false;
   reader->dispatch['\\'] = read_char;
   reader->dispatch['('] = read_vector;
-  reader->dispatch['u'] = read_blob;
+  reader->dispatch['u'] = read_undef_or_blob;
   reader->dispatch['.'] = read_eval;
 
   /* read labels */
diff --git a/extlib/benz/state.c b/extlib/benz/state.c
index aa60da35..a555dc34 100644
--- a/extlib/benz/state.c
+++ b/extlib/benz/state.c
@@ -10,6 +10,7 @@ pic_add_feature(pic_state *pic, const char *feature)
   pic_push(pic, pic_obj_value(pic_intern_cstr(pic, feature)), pic->features);
 }
 
+void pic_init_undef(pic_state *);
 void pic_init_bool(pic_state *);
 void pic_init_pair(pic_state *);
 void pic_init_port(pic_state *);
@@ -107,6 +108,7 @@ pic_init_core(pic_state *pic)
     pic_define_syntactic_keyword(pic, pic->lib->env, pic->sBEGIN, pic->rBEGIN);
     pic_define_syntactic_keyword(pic, pic->lib->env, pic->sDEFINE_SYNTAX, pic->rDEFINE_SYNTAX);
 
+    pic_init_undef(pic); DONE;
     pic_init_bool(pic); DONE;
     pic_init_pair(pic); DONE;
     pic_init_port(pic); DONE;
diff --git a/extlib/benz/undef.c b/extlib/benz/undef.c
new file mode 100644
index 00000000..9e709c0e
--- /dev/null
+++ b/extlib/benz/undef.c
@@ -0,0 +1,21 @@
+/**
+ * See Copyright Notice in picrin.h
+ */
+
+#include "picrin.h"
+
+static pic_value
+pic_undef_undefined_p(pic_state *pic)
+{
+  pic_value v;
+
+  pic_get_args(pic, "o", &v);
+
+  return pic_undef_p(v) ? pic_true_value() : pic_false_value();
+}
+
+void
+pic_init_undef(pic_state *pic)
+{
+  pic_defun(pic, "undefined?", pic_undef_undefined_p);
+}
diff --git a/extlib/benz/write.c b/extlib/benz/write.c
index d2c2a807..32aa3e8c 100644
--- a/extlib/benz/write.c
+++ b/extlib/benz/write.c
@@ -220,7 +220,7 @@ write_core(struct writer_control *p, pic_value obj)
 
   switch (pic_type(obj)) {
   case PIC_TT_UNDEF:
-    xfprintf(file, "#<undef>");
+    xfprintf(file, "#undefined");
     break;
   case PIC_TT_NIL:
     xfprintf(file, "()");