diff --git a/include/mcl/op.hpp b/include/mcl/op.hpp
index 23ee19b..a9df320 100644
--- a/include/mcl/op.hpp
+++ b/include/mcl/op.hpp
@@ -70,6 +70,24 @@ static inline const char *ModeToStr(Mode mode)
 	}
 }
 
+static inline Mode StrToMode(const std::string& s)
+{
+	static const struct {
+		const char *s;
+		Mode mode;
+	} tbl[] = {
+		{ "auto", FP_AUTO },
+		{ "gmp", FP_GMP },
+		{ "llvm", FP_LLVM },
+		{ "llvm_mont", FP_LLVM_MONT },
+		{ "xbyak", FP_XBYAK },
+	};
+	for (size_t i = 0; i < CYBOZU_NUM_OF_ARRAY(tbl); i++) {
+		if (s == tbl[i].s) return tbl[i].mode;
+	}
+	throw cybozu::Exception("StrToMode") << s;
+}
+
 struct Op {
 	mpz_class mp;
 	mcl::SquareRoot sq;