diff --git a/include/mes/lib.h b/include/mes/lib.h
index 6b66107b..b8c70534 100644
--- a/include/mes/lib.h
+++ b/include/mes/lib.h
@@ -52,6 +52,7 @@ int oputc (int c);
 int oputs (char const *s);
 char *search_path (char const *file_name);
 ssize_t _read (int fd, void *buffer, size_t size);
+void assert_msg (int check, char *msg);
 
 extern char *__brk;
 extern void (*__call_at_exit) (void);
diff --git a/simple.make b/simple.make
index 193d7836..87f3fd4c 100644
--- a/simple.make
+++ b/simple.make
@@ -60,6 +60,7 @@ MES_SOURCES =					\
 
 LIB_SOURCES =					\
  lib/mes/eputs.c				\
+ lib/mes/assert_msg.c				\
  lib/mes/itoa.c
 
 M2_SOURCES =					\
diff --git a/src/gc.c b/src/gc.c
index 06e7eb46..3c2fe7c4 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -21,7 +21,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
 #include <errno.h>
 #include <string.h>
 #include <stdlib.h>
@@ -109,7 +108,7 @@ gc_up_arena ()                  /*:((internal)) */
       eputs (":");
       eputs (itoa (ARENA_SIZE - g_free));
       eputs ("\n");
-      assert (0);
+      assert_msg (0, "0");
       exit (1);
     }
   g_cells = p;
@@ -308,7 +307,7 @@ SCM
 gc_push_frame ()                /*:((internal)) */
 {
   if (g_stack < 5)
-    assert (!"STACK FULL");
+    assert_msg (0, "STACK FULL");
   g_stack_array[g_stack - 1] = cell_f;
   g_stack_array[g_stack - 2] = r0;
   g_stack_array[g_stack - 3] = r1;
diff --git a/src/hash.c b/src/hash.c
index 56a8bae4..87b8c5d3 100644
--- a/src/hash.c
+++ b/src/hash.c
@@ -21,7 +21,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
 #include <string.h>
 
 int
@@ -30,7 +29,7 @@ hash_cstring (char const *s, long size)
   int hash = s[0] * 37;
   if (s[0] != 0 && s[1] != 0)
     hash = hash + s[1] * 43;
-  assert (size);
+  assert_msg (size, "size");
   hash = hash % size;
   return hash;
 }
@@ -48,21 +47,21 @@ hash_ (SCM x, long size)
 {
   if (TYPE (x) == TSTRING)
     return hash_cstring (CSTRING (x), size);
-  assert (0);
+  assert_msg (0, "0");
   return hashq_ (x, size);
 }
 
 SCM
 hashq (SCM x, SCM size)
 {
-  assert (0);
+  assert_msg (0, "0");
   return MAKE_NUMBER (hashq_ (x, VALUE (size)));
 }
 
 SCM
 hash (SCM x, SCM size)
 {
-  assert (0);
+  assert_msg (0, "0");
   return MAKE_NUMBER (hash_ (x, VALUE (size)));
 }
 
@@ -237,7 +236,7 @@ make_hash_table (SCM x)
   long size = 0;
   if (TYPE (x) == TPAIR)
     {
-      assert (TYPE (x) == TNUMBER);
+      assert_msg (TYPE (x) == TNUMBER, "TYPE (x) == TNUMBER");
       size = VALUE (x);
     }
   return make_hash_table_ (size);
diff --git a/src/lib.c b/src/lib.c
index 257a526e..638101fa 100644
--- a/src/lib.c
+++ b/src/lib.c
@@ -21,7 +21,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
 #include <stdlib.h>
 
 int g_depth;
@@ -256,7 +255,7 @@ display_error_ (SCM x)
 SCM
 display_port_ (SCM x, SCM p)
 {
-  assert (TYPE (p) == TNUMBER);
+  assert_msg (TYPE (p) == TNUMBER, "TYPE (p) == TNUMBER");
   return fdisplay_ (x, VALUE (p), 0);
 }
 
@@ -277,7 +276,7 @@ write_error_ (SCM x)
 SCM
 write_port_ (SCM x, SCM p)
 {
-  assert (TYPE (p) == TNUMBER);
+  assert_msg (TYPE (p) == TNUMBER, "TYPE (p) == TNUMBER");
   return fdisplay_ (x, VALUE (p), 1);
 }
 
@@ -291,7 +290,7 @@ fdisplay_ (SCM x, int fd, int write_p)  /*:((internal)) */
 SCM
 exit_ (SCM x)                   /*:((name . "exit")) */
 {
-  assert (TYPE (x) == TNUMBER);
+  assert_msg (TYPE (x) == TNUMBER, "TYPE (x) == TNUMBER");
   exit (VALUE (x));
 }
 
diff --git a/src/math.c b/src/math.c
index 8018bdf5..ade8b53e 100644
--- a/src/math.c
+++ b/src/math.c
@@ -21,7 +21,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
 #include <ctype.h>
 #include <limits.h>
 #include <stdio.h>
diff --git a/src/mes.c b/src/mes.c
index 90be5a22..3461824f 100644
--- a/src/mes.c
+++ b/src/mes.c
@@ -21,7 +21,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
 #include <fcntl.h>
 #include <limits.h>
 #include <stdlib.h>
@@ -36,7 +35,7 @@ alloc (long n)
   SCM x = g_free;
   g_free = g_free + n;
   if (g_free > ARENA_SIZE)
-    assert (!"alloc: out of memory");
+    assert_msg (0, "alloc: out of memory");
   return x;
 }
 
@@ -53,7 +52,7 @@ make_cell__ (long type, SCM car, SCM cdr)
 SCM
 make_cell_ (SCM type, SCM car, SCM cdr)
 {
-  assert (TYPE (type) == TNUMBER);
+  assert_msg (TYPE (type) == TNUMBER, "TYPE (type) == TNUMBER");
   long t = VALUE (type);
   if (t == TCHAR || t == TNUMBER)
     {
@@ -233,7 +232,7 @@ error (SCM key, SCM x)
   eputs (": ");
   write_error_ (x);
   eputs ("\n");
-  assert (0);
+  assert_msg (0, "0");
   exit (1);
 }
 
@@ -1108,7 +1107,7 @@ begin_expand:
               else if (TYPE (r1) == TPORT)
                 input = set_current_input_port (r1);
               else
-                assert (0);
+                assert_msg (0, "0");
 
               push_cc (input, r2, r0, cell_vm_return);
               x = read_input_file_env (r0);
diff --git a/src/module.c b/src/module.c
index 683e255a..9e13c478 100644
--- a/src/module.c
+++ b/src/module.c
@@ -21,8 +21,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
-
 SCM
 make_module_type ()             /*:(internal)) */
 {
diff --git a/src/posix.c b/src/posix.c
index 9ec54ea9..4366ed06 100644
--- a/src/posix.c
+++ b/src/posix.c
@@ -21,7 +21,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
 #include <fcntl.h>
 #include <limits.h>
 #include <stdlib.h>
@@ -147,7 +146,7 @@ write_byte (SCM x)              /*:((arity . n)) */
   char cc = VALUE (c);
   write (fd, &cc, 1);
 #if !__MESC__
-  assert (TYPE (c) == TNUMBER || TYPE (c) == TCHAR);
+  assert_msg (TYPE (c) == TNUMBER || TYPE (c) == TCHAR, "TYPE (c) == TNUMBER || TYPE (c) == TCHAR");
 #endif
   return c;
 }
@@ -298,7 +297,7 @@ execl_ (SCM file_name, SCM args)        /*:((name . "execl")) */
   i = i + 1;
   while (args != cell_nil)
     {
-      assert (TYPE (CAR (args)) == TSTRING);
+      assert_msg (TYPE (CAR (args)) == TSTRING, "TYPE (CAR (args)) == TSTRING");
       c_argv[i] = CSTRING (CAR (args));
       i = i + 1;
       args = CDR (args);
diff --git a/src/reader.c b/src/reader.c
index aa1768be..60112bb4 100644
--- a/src/reader.c
+++ b/src/reader.c
@@ -22,7 +22,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
 #include <ctype.h>
 #include <stdio.h>
 #include <string.h>
diff --git a/src/string.c b/src/string.c
index 4fdc1885..4e886902 100644
--- a/src/string.c
+++ b/src/string.c
@@ -21,8 +21,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
-
 #include <limits.h>
 #include <string.h>
 
@@ -115,7 +113,7 @@ string_equal_p (SCM a, SCM b)   /*:((name . "string=?")) */
       eputs ("b= ");
       write_error_ (b);
       eputs ("\n");
-      assert ((TYPE (a) == TSTRING && TYPE (b) == TSTRING) || (TYPE (a) == TKEYWORD || TYPE (b) == TKEYWORD));
+      assert_msg ((TYPE (a) == TSTRING && TYPE (b) == TSTRING) || (TYPE (a) == TKEYWORD || TYPE (b) == TKEYWORD), "(TYPE (a) == TSTRING && TYPE (b) == TSTRING) || (TYPE (a) == TKEYWORD || TYPE (b) == TKEYWORD)");
     }
   if (a == b
       || STRING (a) == STRING (b)
@@ -231,7 +229,7 @@ string_append (SCM x)           /*:((arity . n)) */
   while (x != cell_nil)
     {
       SCM string = CAR (x);
-      assert (TYPE (string) == TSTRING);
+      assert_msg (TYPE (string) == TSTRING, "TYPE (string) == TSTRING");
       memcpy (p, CSTRING (string), LENGTH (string) + 1);
       p = p + LENGTH (string);
       size = size + LENGTH (string);
@@ -245,15 +243,15 @@ string_append (SCM x)           /*:((arity . n)) */
 SCM
 string_length (SCM string)
 {
-  assert (TYPE (string) == TSTRING);
+  assert_msg (TYPE (string) == TSTRING, "TYPE (string) == TSTRING");
   return MAKE_NUMBER (LENGTH (string));
 }
 
 SCM
 string_ref (SCM str, SCM k)
 {
-  assert (TYPE (str) == TSTRING);
-  assert (TYPE (k) == TNUMBER);
+  assert_msg (TYPE (str) == TSTRING, "TYPE (str) == TSTRING");
+  assert_msg (TYPE (k) == TNUMBER, "TYPE (k) == TNUMBER");
   size_t size = LENGTH (str);
   size_t i = VALUE (k);
   if (i > size)
diff --git a/src/struct.c b/src/struct.c
index 16f94e75..522328b4 100644
--- a/src/struct.c
+++ b/src/struct.c
@@ -21,8 +21,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
-
 SCM
 make_struct (SCM type, SCM fields, SCM printer)
 {
@@ -57,15 +55,15 @@ make_struct (SCM type, SCM fields, SCM printer)
 SCM
 struct_length (SCM x)
 {
-  assert (TYPE (x) == TSTRUCT);
+  assert_msg (TYPE (x) == TSTRUCT, "TYPE (x) == TSTRUCT");
   return MAKE_NUMBER (LENGTH (x));
 }
 
 SCM
 struct_ref_ (SCM x, long i)
 {
-  assert (TYPE (x) == TSTRUCT);
-  assert (i < LENGTH (x));
+  assert_msg (TYPE (x) == TSTRUCT, "TYPE (x) == TSTRUCT");
+  assert_msg (i < LENGTH (x), "i < LENGTH (x)");
   SCM e = STRUCT (x) + i;
   if (TYPE (e) == TREF)
     e = REF (e);
@@ -79,8 +77,8 @@ struct_ref_ (SCM x, long i)
 SCM
 struct_set_x_ (SCM x, long i, SCM e)
 {
-  assert (TYPE (x) == TSTRUCT);
-  assert (i < LENGTH (x));
+  assert_msg (TYPE (x) == TSTRUCT, "TYPE (x) == TSTRUCT");
+  assert_msg (i < LENGTH (x), "i < LENGTH (x)");
   g_cells[STRUCT (x) + i] = g_cells[vector_entry (e)];
   return cell_unspecified;
 }
diff --git a/src/vector.c b/src/vector.c
index d3ee94fc..ab206ab4 100644
--- a/src/vector.c
+++ b/src/vector.c
@@ -21,8 +21,6 @@
 #include "mes/lib.h"
 #include "mes/mes.h"
 
-#include <assert.h>
-
 SCM
 make_vector__ (long k)
 {
@@ -43,15 +41,15 @@ make_vector_ (SCM n)
 SCM
 vector_length (SCM x)
 {
-  assert (TYPE (x) == TVECTOR);
+  assert_msg (TYPE (x) == TVECTOR, "TYPE (x) == TVECTOR");
   return MAKE_NUMBER (LENGTH (x));
 }
 
 SCM
 vector_ref_ (SCM x, long i)
 {
-  assert (TYPE (x) == TVECTOR);
-  assert (i < LENGTH (x));
+  assert_msg (TYPE (x) == TVECTOR, "TYPE (x) == TVECTOR");
+  assert_msg (i < LENGTH (x), "i < LENGTH (x)");
   SCM e = VECTOR (x) + i;
   if (TYPE (e) == TREF)
     e = REF (e);
@@ -79,8 +77,8 @@ vector_entry (SCM x)
 SCM
 vector_set_x_ (SCM x, long i, SCM e)
 {
-  assert (TYPE (x) == TVECTOR);
-  assert (i < LENGTH (x));
+  assert_msg (TYPE (x) == TVECTOR, "TYPE (x) == TVECTOR");
+  assert_msg (i < LENGTH (x), "i < LENGTH (x)");
   g_cells[VECTOR (x) + i] = g_cells[vector_entry (e)];
   return cell_unspecified;
 }