diff --git a/include/mcl/fp_tower.hpp b/include/mcl/fp_tower.hpp
index c311aae..f79cba7 100644
--- a/include/mcl/fp_tower.hpp
+++ b/include/mcl/fp_tower.hpp
@@ -16,6 +16,7 @@ class FpDblT : public fp::Serializable<FpDblT<Fp> > {
 	Unit v_[Fp::maxSize * 2];
 public:
 	static size_t getUnitSize() { return Fp::op_.N * 2; }
+	const fp::Unit *getUnit() const { return v_; }
 	void dump() const
 	{
 		const size_t n = getUnitSize();
@@ -662,25 +663,26 @@ struct Fp2DblT {
 		FpDbl::neg(y.a, x.a);
 		FpDbl::neg(y.b, x.b);
 	}
-	static void mul_xi(Fp2DblT& y, const Fp2DblT& x)
+	static void mul_xi_1C(Fp2DblT& y, const Fp2DblT& x)
+	{
+		FpDbl t;
+		FpDbl::add(t, x.a, x.b);
+		FpDbl::sub(y.a, x.a, x.b);
+		y.b = t;
+	}
+	static void mul_xi_genericC(Fp2DblT& y, const Fp2DblT& x)
 	{
 		const uint32_t xi_a = Fp2::get_xi_a();
-		if (xi_a == 1) {
-			FpDbl t;
-			FpDbl::add(t, x.a, x.b);
-			FpDbl::sub(y.a, x.a, x.b);
-			y.b = t;
-		} else {
-			FpDbl t;
-			FpDbl::mulUnit(t, x.a, xi_a);
-			FpDbl::sub(t, t, x.b);
-			FpDbl::mulUnit(y.b, x.b, xi_a);
-			FpDbl::add(y.b, y.b, x.a);
-			y.a = t;
-		}
+		FpDbl t;
+		FpDbl::mulUnit(t, x.a, xi_a);
+		FpDbl::sub(t, t, x.b);
+		FpDbl::mulUnit(y.b, x.b, xi_a);
+		FpDbl::add(y.b, y.b, x.a);
+		y.a = t;
 	}
 	static void (*mulPre)(Fp2DblT&, const Fp2&, const Fp2&);
 	static void (*sqrPre)(Fp2DblT&, const Fp2&);
+	static void (*mul_xi)(Fp2DblT&, const Fp2DblT&);
 	static void mod(Fp2& y, const Fp2DblT& x)
 	{
 		FpDbl::mod(y.a, x.a);
@@ -716,6 +718,18 @@ struct Fp2DblT {
 				sqrPre = fp2Dbl_sqrPreW<false>;
 			}
 		}
+		const uint32_t xi_a = Fp2::get_xi_a();
+		switch (xi_a) {
+		case 1:
+			mul_xi = mul_xi_1C;
+			if (op.fp2Dbl_mul_xiA_) {
+				mul_xi = fp::func_ptr_cast<void (*)(Fp2DblT&, const Fp2DblT&)>(op.fp2Dbl_mul_xiA_);
+			}
+			break;
+		default:
+			mul_xi = mul_xi_genericC;
+			break;
+		}
 	}
 	/*
 		Fp2Dbl::mulPre by FpDblT
@@ -770,6 +784,7 @@ struct Fp2DblT {
 
 template<class Fp> void (*Fp2DblT<Fp>::mulPre)(Fp2DblT&, const Fp2T<Fp>&, const Fp2T<Fp>&);
 template<class Fp> void (*Fp2DblT<Fp>::sqrPre)(Fp2DblT&, const Fp2T<Fp>&);
+template<class Fp> void (*Fp2DblT<Fp>::mul_xi)(Fp2DblT<Fp>&, const Fp2DblT<Fp>&);
 
 template<class Fp> Fp2T<Fp> Fp2T<Fp>::g[Fp2T<Fp>::gN];
 template<class Fp> Fp2T<Fp> Fp2T<Fp>::g2[Fp2T<Fp>::gN];
diff --git a/include/mcl/op.hpp b/include/mcl/op.hpp
index 7e314ae..76cf7f0 100644
--- a/include/mcl/op.hpp
+++ b/include/mcl/op.hpp
@@ -226,6 +226,7 @@ struct Op {
 	void2u fpDbl_modA_;
 	void3u fp2Dbl_mulPreA_;
 	void2u fp2Dbl_sqrPreA_;
+	void2u fp2Dbl_mul_xiA_;
 	size_t maxN;
 	size_t N;
 	size_t bitSize;
@@ -314,6 +315,7 @@ struct Op {
 		fpDbl_modA_ = 0;
 		fp2Dbl_mulPreA_ = 0;
 		fp2Dbl_sqrPreA_ = 0;
+		fp2Dbl_mul_xiA_ = 0;
 		maxN = 0;
 		N = 0;
 		bitSize = 0;
diff --git a/src/fp_generator.hpp b/src/fp_generator.hpp
index f5e34cf..cfbb53f 100644
--- a/src/fp_generator.hpp
+++ b/src/fp_generator.hpp
@@ -457,6 +457,10 @@ private:
 		op.fp2Dbl_sqrPreA_ = gen_fp2Dbl_sqrPre();
 		if (op.fp2Dbl_sqrPreA_) setFuncInfo(prof_, suf, "2Dbl_sqrPre", op.fp2Dbl_sqrPreA_, getCurr());
 
+		align(16);
+		op.fp2Dbl_mul_xiA_ = gen_fp2Dbl_mul_xi();
+		if (op.fp2Dbl_mul_xiA_) setFuncInfo(prof_, suf, "2Dbl_mul_xi", op.fp2Dbl_mul_xiA_, getCurr());
+
 		align(16);
 		op.fp2_mulA_ = gen_fp2_mul();
 		setFuncInfo(prof_, suf, "2_mul", op.fp2_mulA_, getCurr());
@@ -3173,6 +3177,13 @@ private:
 			}
 		}
 	}
+	// y[i] &= t
+	void andPack(const Pack& y, const Reg64& t)
+	{
+		for (int i = 0; i < (int)y.size(); i++) {
+			and_(y[i], t);
+		}
+	}
 	/*
 		[rdx:x:t0] <- py[1:0] * x
 		destroy x, t
@@ -3647,6 +3658,55 @@ private:
 		call(mulPreL);
 		return func;
 	}
+	void2u gen_fp2Dbl_mul_xi()
+	{
+		if (isFullBit_) return 0;
+		if (op_->xi_a != 1) return 0;
+		void2u func = getCurr<void2u>();
+		// y = (x.a - x.b, x.a + x.b)
+		StackFrame sf(this, 2, pn_ * 2, FpByte_ * 2);
+		Pack t1 = sf.t.sub(0, pn_);
+		Pack t2 = sf.t.sub(pn_, pn_);
+		const RegExp& ya = sf.p[0];
+		const RegExp& yb = sf.p[0] + FpByte_ * 2;
+		const RegExp& xa = sf.p[1];
+		const RegExp& xb = sf.p[1] + FpByte_ * 2;
+		// [rsp] = x.a + x.b
+		for (int i = 0; i < pn_ * 2; i++) {
+			mov(rax, ptr[xa + i * 8]);
+			if (i == 0) {
+				add(rax, ptr[xb + i * 8]);
+			} else {
+				adc(rax, ptr[xb + i * 8]);
+			}
+			mov(ptr[rsp + i * 8], rax);
+		}
+		// low : x.a =  x.a - x.b
+		load_rm(t1, xa);
+		sub_rm(t1, xb);
+		store_mr(ya, t1);
+		// high : x.a = (x.a - x.b) % p
+		load_rm(t1, xa + FpByte_);
+		sub_rm(t1, xb + FpByte_, true);
+		lea(rax, ptr[rip + pL_]);
+		load_rm(t2, rax); // t2 = p
+		sbb(rax, rax);
+		andPack(t2, rax);
+		add_rr(t1, t2); // mod p
+		store_mr(ya + FpByte_, t1);
+
+		// low : y.b = [rsp]
+		for (int i = 0; i < pn_; i++) {
+			mov(rax, ptr[rsp + i * 8]);
+			mov(ptr[yb + i * 8], rax);
+		}
+		// high : y.b = (x.a + x.b) % p
+		load_rm(t1, rsp + FpByte_);
+		lea(rax, ptr[rip + pL_]);
+		sub_p_mod(t2, t1, rax);
+		store_mr(yb + FpByte_, t2);
+		return func;
+	}
 	void gen_fp2_add4()
 	{
 		assert(!isFullBit_);
diff --git a/src/fp_static_code.hpp b/src/fp_static_code.hpp
index d396526..ef27f1b 100644
--- a/src/fp_static_code.hpp
+++ b/src/fp_static_code.hpp
@@ -38,6 +38,7 @@ void mclx_Fp2_mul(Unit*, const Unit*, const Unit*);
 void mclx_Fp2_sqr(Unit*, const Unit*);
 void mclx_Fp2_mul2(Unit*, const Unit*);
 void mclx_Fp2_mul_xi(Unit*, const Unit*);
+void mclx_Fp2Dbl_mul_xi(Unit*, const Unit*);
 
 Unit mclx_Fr_addPre(Unit*, const Unit*, const Unit*);
 Unit mclx_Fr_subPre(Unit*, const Unit*, const Unit*);
@@ -79,6 +80,7 @@ void setStaticCode(mcl::fp::Op& op)
 		op.fp2_sqrA_ = mclx_Fp2_sqr;
 		op.fp2_mul2A_ = mclx_Fp2_mul2;
 		op.fp2_mul_xiA_ = mclx_Fp2_mul_xi;
+		op.fp2Dbl_mul_xiA_ = mclx_Fp2Dbl_mul_xi;
 		op.fp_preInv = mclx_Fp_preInv;
 	} else {
 		// Fr, sizeof(Fr) = 32
diff --git a/test/common_test.hpp b/test/common_test.hpp
index afe23ee..0781ebb 100644
--- a/test/common_test.hpp
+++ b/test/common_test.hpp
@@ -161,8 +161,31 @@ void testABCD()
 	}
 }
 
+void testFp2Dbl_mul_xi1()
+{
+	if (Fp2::get_xi_a() != 1) return;
+	puts("testFp2Dbl_mul_xi1");
+	cybozu::XorShift rg;
+	for (int i = 0; i < 100; i++) {
+		Fp a1, a2;
+		a1.setByCSPRNG(rg);
+		a2.setByCSPRNG(rg);
+		Fp2Dbl x;
+		FpDbl::mulPre(x.a, a1, a2);
+		a1.setByCSPRNG(rg);
+		a2.setByCSPRNG(rg);
+		FpDbl::mulPre(x.b, a1, a2);
+		Fp2Dbl ok;
+		Fp2Dbl::mul_xi_1C(x, x);
+		Fp2Dbl::mul_xi(x, x);
+		CYBOZU_TEST_EQUAL_ARRAY(ok.a.getUnit(), x.a.getUnit(), ok.a.getUnitSize());
+		CYBOZU_TEST_EQUAL_ARRAY(ok.b.getUnit(), x.b.getUnit(), ok.b.getUnitSize());
+	}
+}
+
 void testCommon(const G1& P, const G2& Q)
 {
+	testFp2Dbl_mul_xi1();
 	testABCD();
 	testMul2();
 	puts("G1");