diff --git a/include/mcl/bn.hpp b/include/mcl/bn.hpp
index f1188c8..74eace9 100644
--- a/include/mcl/bn.hpp
+++ b/include/mcl/bn.hpp
@@ -475,10 +475,10 @@ struct MapTo {
 		Frobenius2(T1, T1);
 		G2::add(Q, T0, T1);
 	}
-	void mulByCofactorBLS12(G2& Q, const G2& P) const
+	void mulByCofactorBLS12(G2& Q, const G2& P, bool fast = false) const
 	{
 		mulByCofactorBLS12fast(Q, P);
-		if (useOriginalG2cofactor_) {
+		if (useOriginalG2cofactor_ && !fast) {
 			Q *= g2cofactorAdj_;
 			return;
 		}
@@ -588,14 +588,14 @@ struct MapTo {
 		}
 		assert(P.isValid());
 	}
-	void mulByCofactor(G2& P) const
+	void mulByCofactor(G2& P, bool fast = false) const
 	{
 		switch(type_) {
 		case BNtype:
 			mulByCofactorBN(P, P);
 			break;
 		case BLS12type:
-			mulByCofactorBLS12(P, P);
+			mulByCofactorBLS12(P, P, fast);
 			break;
 		}
 		assert(P.isValid());
@@ -606,7 +606,7 @@ struct MapTo {
 		mulByCofactor(P);
 		return true;
 	}
-	bool calc(G2& P, const Fp2& t) const
+	bool calc(G2& P, const Fp2& t, bool fast = false) const
 	{
 		if (!mapToEc(P, t)) return false;
 		if (mapToMode_ == MCL_MAP_TO_MODE_ETH2) {
@@ -617,7 +617,7 @@ struct MapTo {
 				P.y = negY;
 			}
 		}
-		mulByCofactor(P);
+		mulByCofactor(P, fast);
 		return true;
 	}
 };
@@ -2141,7 +2141,7 @@ inline bool setMapToMode(int mode)
 	return BN::nonConstParam.mapTo.setMapToMode(mode);
 }
 inline void mapToG1(bool *pb, G1& P, const Fp& x) { *pb = BN::param.mapTo.calc(P, x); }
-inline void mapToG2(bool *pb, G2& P, const Fp2& x) { *pb = BN::param.mapTo.calc(P, x); }
+inline void mapToG2(bool *pb, G2& P, const Fp2& x, bool fast = false) { *pb = BN::param.mapTo.calc(P, x, fast); }
 #ifndef CYBOZU_DONT_USE_EXCEPTION
 inline void mapToG1(G1& P, const Fp& x)
 {
@@ -2149,10 +2149,10 @@ inline void mapToG1(G1& P, const Fp& x)
 	mapToG1(&b, P, x);
 	if (!b) throw cybozu::Exception("mapToG1:bad value") << x;
 }
-inline void mapToG2(G2& P, const Fp2& x)
+inline void mapToG2(G2& P, const Fp2& x, bool fast = false)
 {
 	bool b;
-	mapToG2(&b, P, x);
+	mapToG2(&b, P, x, fast);
 	if (!b) throw cybozu::Exception("mapToG2:bad value") << x;
 }
 #endif
diff --git a/test/bls12_test.cpp b/test/bls12_test.cpp
index 3f056af..c967006 100644
--- a/test/bls12_test.cpp
+++ b/test/bls12_test.cpp
@@ -730,9 +730,8 @@ CYBOZU_TEST_AUTO(eth2)
 	const char *expectSig = "b9d1bf921b3dd048bdce38c2ceac2a2a8093c864881f2415f22b198de935ffa791707855c1656dc21a7af2d502bb46590151d645f062634c3b2cb79c4ed1c4a4b8b3f19f0f5c76965c651553e83d153ff95353735156eff77692f7a62ae653fb";
 	CYBOZU_TEST_EQUAL(sig.getStr(mcl::IoSerializeHexStr), expectSig);
 
-	CYBOZU_BENCH_C("mapToG2  org-cofactor", 1000, mapToG2, Q, m);
-	setOriginalG2cofactor(false);
-	CYBOZU_BENCH_C("mapToG2 fast-cofactor", 1000, mapToG2, Q, m);
+	CYBOZU_BENCH_C("mapToG2  org-cofactor", 1000, mapToG2, Q, m, false);
+	CYBOZU_BENCH_C("mapToG2 fast-cofactor", 1000, mapToG2, Q, m, true);
 
 	Fp2 x;
 	x.a = 5;
@@ -743,7 +742,7 @@ CYBOZU_TEST_AUTO(eth2)
 	BN::param.mapTo.mapToEc(Q, x);
 	G2::mulGeneric(Q1, Q, g2c);
 	Q2 = Q;
-	BN::param.mapTo.mulByCofactor(Q2);
+	BN::param.mapTo.mulByCofactor(Q2, true);
 	Q2 *= g2ca;
 	CYBOZU_TEST_EQUAL(Q1, Q2);
 }