diff --git a/include/mcl/conversion.hpp b/include/mcl/conversion.hpp
index 5ba7de0..798a549 100644
--- a/include/mcl/conversion.hpp
+++ b/include/mcl/conversion.hpp
@@ -451,17 +451,16 @@ size_t strToArray(bool *pIsMinus, UT *x, size_t xN, const char *buf, size_t bufS
 	return true if success else flase
 */
 template<class OutputStream>
-bool writeHexStr(OutputStream& os, const void *src, size_t n)
+void writeHexStr(bool *pb, OutputStream& os, const void *src, size_t n)
 {
-	bool b;
 	const uint8_t *p = (const uint8_t *)src;
 	for (size_t i = 0; i < n; i++) {
 		char hex[2];
 		cybozu::itohex(hex, sizeof(hex), p[i], false);
-		cybozu::write(&b, os, hex, sizeof(hex));
-		if (!b) return false;
+		cybozu::write(pb, os, hex, sizeof(hex));
+		if (!*pb) return;
 	}
-	return true;
+	*pb = true;
 }
 /*
 	read hex string from is and convert it to byte array
diff --git a/include/mcl/ec.hpp b/include/mcl/ec.hpp
index 9b09486..ec46602 100644
--- a/include/mcl/ec.hpp
+++ b/include/mcl/ec.hpp
@@ -713,7 +713,7 @@ public:
 				}
 			}
 			if (ioMode & IoSerializeHexStr) {
-				*pb = mcl::fp::writeHexStr(os, buf, n + adj);
+				mcl::fp::writeHexStr(pb, os, buf, n + adj);
 			} else {
 				cybozu::write(pb, os, buf, n + adj);
 			}
diff --git a/include/mcl/fp.hpp b/include/mcl/fp.hpp
index ca3feae..4850012 100644
--- a/include/mcl/fp.hpp
+++ b/include/mcl/fp.hpp
@@ -278,7 +278,7 @@ public:
 				fp::Block b;
 				getBlock(b);
 				if (ioMode & IoSerializeHexStr) {
-					*pb = mcl::fp::writeHexStr(os, b.p, n);
+					mcl::fp::writeHexStr(pb, os, b.p, n);
 				} else {
 					cybozu::write(pb, os, b.p, n);
 				}
diff --git a/test/conversion_test.cpp b/test/conversion_test.cpp
index 06f31af..ec11fe9 100644
--- a/test/conversion_test.cpp
+++ b/test/conversion_test.cpp
@@ -82,7 +82,9 @@ CYBOZU_TEST_AUTO(writeHexStr)
 		const char *bin = tbl[i].bin;
 		const char *hex = tbl[i].hex;
 		size_t n = tbl[i].n;
-		CYBOZU_TEST_ASSERT(mcl::fp::writeHexStr(os, bin, n));
+		bool b;
+		mcl::fp::writeHexStr(&b, os, bin, n);
+		CYBOZU_TEST_ASSERT(b);
 		CYBOZU_TEST_EQUAL(os.getPos(), n * 2);
 		CYBOZU_TEST_EQUAL_ARRAY(buf, hex, n * 2);