diff --git a/include/mcl/conversion.hpp b/include/mcl/conversion.hpp
index 6fbd394..2383efd 100644
--- a/include/mcl/conversion.hpp
+++ b/include/mcl/conversion.hpp
@@ -5,7 +5,7 @@
 #include <mcl/util.hpp>
 /**
 	@file
-	@brief utility of Fp
+	@brief convertion from T[] to str2, str16
 	@author MITSUNARI Shigeo(@herumi)
 	@license modified new BSD license
 	http://opensource.org/licenses/BSD-3-Clause
diff --git a/include/mcl/fp.hpp b/include/mcl/fp.hpp
index 43e8a1b..eccef53 100644
--- a/include/mcl/fp.hpp
+++ b/include/mcl/fp.hpp
@@ -20,6 +20,7 @@
 //	#define USE_MONT_FP
 #endif
 #include <cybozu/hash.hpp>
+#include <cybozu/bit_operation.hpp>
 #include <mcl/op.hpp>
 #include <mcl/util.hpp>
 #include <mcl/power.hpp>
@@ -240,11 +241,15 @@ public:
 		FpT t(x);
 		for (size_t i = 0; i < yn; i++) {
 			const Unit v = y[i];
-			int m = (int)sizeof(Unit) * 8;
+			int m = (int)fp::UnitBitN;
 			if (i == yn - 1) {
+#if 1
+				m = v ? cybozu::bsr<Unit>(v) + 1 : 0;
+#else
 				while (m > 0 && (v & (Unit(1) << (m - 1))) == 0) {
 					m--;
 				}
+#endif
 			}
 			for (int j = 0; j < m; j++) {
 				if (v & (Unit(1) << j)) {
diff --git a/include/mcl/op.hpp b/include/mcl/op.hpp
index e1b92a3..61fd548 100644
--- a/include/mcl/op.hpp
+++ b/include/mcl/op.hpp
@@ -6,7 +6,6 @@
 	@license modified new BSD license
 	http://opensource.org/licenses/BSD-3-Clause
 */
-#include <assert.h>
 #include <mcl/gmp_util.hpp>
 
 #ifndef MCL_MAX_OP_BIT_N
diff --git a/include/mcl/util.hpp b/include/mcl/util.hpp
index 4acb6d1..cd21ae4 100644
--- a/include/mcl/util.hpp
+++ b/include/mcl/util.hpp
@@ -1,7 +1,7 @@
 #pragma once
 /**
 	@file
-	@brief definition of Unit and some functions
+	@brief functions for T[]
 	@author MITSUNARI Shigeo(@herumi)
 	@license modified new BSD license
 	http://opensource.org/licenses/BSD-3-Clause