diff --git a/ChangeLog b/ChangeLog
index 3060843..5941b72 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2013-06-18  Niibe Yutaka  <gniibe@fsij.org>
+
+	* src/modp256.c (p256): Add const qualifier.
+
 2013-03-19  Niibe Yutaka  <gniibe@fsij.org>
 
 	* src/random.c (random_gen): New (was: random_byte).
diff --git a/src/modp256.c b/src/modp256.c
index 69ced35..201179c 100644
--- a/src/modp256.c
+++ b/src/modp256.c
@@ -43,8 +43,8 @@
 2^256 - 2^224 + 2^192 + 2^96 - 1
   0 ffffffff 00000001 00000000 00000000 00000000 ffffffff ffffffff ffffffff
 */
-bn256 p256 = { {0xffffffff, 0xffffffff, 0xffffffff, 0x00000000,
-		0x00000000, 0x00000000, 0x00000001, 0xffffffff} };
+const bn256 p256 = { {0xffffffff, 0xffffffff, 0xffffffff, 0x00000000,
+		      0x00000000, 0x00000000, 0x00000001, 0xffffffff} };
 
 /**
  * @brief  X = (A + B) mod p256
diff --git a/src/modp256.h b/src/modp256.h
index 3c63d80..ee88a65 100644
--- a/src/modp256.h
+++ b/src/modp256.h
@@ -1,4 +1,4 @@
-extern bn256 p256;
+extern const bn256 p256;
 #define P256 (&p256)
 
 void modp256_add (bn256 *X, const bn256 *A, const bn256 *B);