diff --git a/include/mcl/fp.hpp b/include/mcl/fp.hpp
index e2a135c..f50b74d 100644
--- a/include/mcl/fp.hpp
+++ b/include/mcl/fp.hpp
@@ -36,8 +36,6 @@ struct ZnTag;
 
 namespace fp {
 
-void arrayToStr(std::string& str, const Unit *x, size_t n, int base, bool withPrefix);
-
 // copy src to dst as little endian
 void copyUnitToByteAsLE(uint8_t *dst, const Unit *src, size_t byteSize);
 // copy src to dst as little endian
@@ -275,8 +273,8 @@ public:
 			fp::copyUnitToByteAsLE(reinterpret_cast<uint8_t*>(&str[0]), p, str.size());
 			return;
 		}
-		// use low [1-4]-bit ioMode for base
-		fp::arrayToStr(str, b.p, b.n, ioMode & 30, (ioMode & IoPrefix) != 0);
+		// use low 5-bit ioMode for base
+		fp::arrayToStr(str, b.p, b.n, ioMode & 31);
 	}
 	std::string getStr(int ioMode = 0) const
 	{
diff --git a/include/mcl/op.hpp b/include/mcl/op.hpp
index 2f99c42..7b2ec14 100644
--- a/include/mcl/op.hpp
+++ b/include/mcl/op.hpp
@@ -291,9 +291,12 @@ private:
 /*
 	read data from is according to ioMode,
 	and set x[0, n) with abs(buf[0, bufSize/sizeof(Unit)))
+	@note byteSize is not num of Unit
 */
 void streamToArray(bool *pIsMinus, Unit *x, size_t byteSize, std::istream& is, int ioMode);
 
+void arrayToStr(std::string& str, const Unit *x, size_t n, int ioMode);
+
 inline const char* getIoSeparator(int ioMode)
 {
 	return (ioMode & (IoArray | IoArrayRaw | IoTight)) ? "" : " ";
diff --git a/src/fp.cpp b/src/fp.cpp
index 76dada3..c99a3fe 100644
--- a/src/fp.cpp
+++ b/src/fp.cpp
@@ -458,8 +458,10 @@ void Op::init(const std::string& mstr, size_t maxBitSize, Mode mode)
 	sq.set(mp);
 }
 
-void arrayToStr(std::string& str, const Unit *x, size_t n, int base, bool withPrefix)
+void arrayToStr(std::string& str, const Unit *x, size_t n, int ioMode)
 {
+	int base = ioMode & ~IoPrefix;
+	bool withPrefix = ioMode & IoPrefix;
 	switch (base) {
 	case 0:
 	case 10:
@@ -592,7 +594,7 @@ uint64_t getUint64(bool *pb, const fp::Block& b)
 	}
 	if (!pb) {
 		std::string str;
-		arrayToStr(str, b.p, b.n, 10, false);
+		arrayToStr(str, b.p, b.n, 10);
 		throw cybozu::Exception("fp::getUint64:large value") << str;
 	}
 	*pb = false;
@@ -631,7 +633,7 @@ int64_t getInt64(bool *pb, fp::Block& b, const fp::Op& op)
 	}
 	if (!pb) {
 		std::string str;
-		arrayToStr(str, b.p, b.n, 10, false);
+		arrayToStr(str, b.p, b.n, 10);
 		throw cybozu::Exception("fp::getInt64:large value") << str << isNegative;
 	}
 	*pb = false;