diff --git a/include/mcl/fp.hpp b/include/mcl/fp.hpp
index 96110a8..4f3c35b 100644
--- a/include/mcl/fp.hpp
+++ b/include/mcl/fp.hpp
@@ -77,8 +77,8 @@ void hkdf_extract_addZeroByte(uint8_t hmac[32], const uint8_t *salt, size_t salt
 void hkdf_extract(uint8_t hmac[32], const uint8_t *salt, size_t saltSize, const uint8_t *msg, size_t msgSize);
 void hkdf_expand(uint8_t out[64], const uint8_t prk[32], char info[6]);
 
-// draft-07
-void expand_message_xmd(uint8_t out[256], const void *msg, size_t msgSize, const void *dst, size_t dstSize);
+// draft-07 outSize = 128 or 256
+void expand_message_xmd(uint8_t out[], size_t outSize, const void *msg, size_t msgSize, const void *dst, size_t dstSize);
 
 void expand_message_xmd06(uint8_t out[256], const void *msg, size_t msgSize, const void *dst, size_t dstSize);
 
diff --git a/include/mcl/mapto_wb19.hpp b/include/mcl/mapto_wb19.hpp
index dd11e09..bb7df52 100644
--- a/include/mcl/mapto_wb19.hpp
+++ b/include/mcl/mapto_wb19.hpp
@@ -445,7 +445,7 @@ struct MapToG2_WB19 {
 		if (draftVersion_ == 6) {
 			mcl::fp::expand_message_xmd06(md, msg, msgSize, dst, dstSize);
 		} else {
-			mcl::fp::expand_message_xmd(md, msg, msgSize, dst, dstSize);
+			mcl::fp::expand_message_xmd(md, sizeof(md), msg, msgSize, dst, dstSize);
 		}
 		Fp *x = out[0].getFp0();
 		for (size_t i = 0; i < 4; i++) {
diff --git a/src/fp.cpp b/src/fp.cpp
index f9307ac..343d242 100644
--- a/src/fp.cpp
+++ b/src/fp.cpp
@@ -194,20 +194,21 @@ void expand_message_xmd06(uint8_t out[256], const void *msg, size_t msgSize, con
 	}
 }
 
-void expand_message_xmd(uint8_t out[256], const void *msg, size_t msgSize, const void *dst, size_t dstSize)
+void expand_message_xmd(uint8_t out[], size_t outSize, const void *msg, size_t msgSize, const void *dst, size_t dstSize)
 {
-	const size_t len_in_bytes = 256;
+	assert(outSize == 128 || outSize == 256);
 	const size_t mdSize = 32;
 	const size_t r_in_bytes = 64;
-	const size_t ell = len_in_bytes / mdSize;
+	const size_t n = outSize / mdSize;
 	static const uint8_t Z_pad[r_in_bytes] = {};
 	assert(dstSize < 256);
 	/*
-		Z_apd | msg | BE(len_in_bytes, 2) | BE(0, 1) | DST | BE(dstSize, 1)
+		Z_apd | msg | BE(outSize, 2) | BE(0, 1) | DST | BE(dstSize, 1)
 	*/
-	static const uint8_t lenBuf[2] = { 1, 0 }; // 256 = len_in_bytes
+	uint8_t lenBuf[2] = { 1, 0 }; // 256 = outSize
 	uint8_t iBuf = 0;
 	uint8_t dstSizeBuf = uint8_t(dstSize);
+	cybozu::Set16bitAsBE(lenBuf, uint16_t(outSize));
 	cybozu::Sha256 h;
 	h.update(Z_pad, r_in_bytes);
 	h.update(msg, msgSize);
@@ -223,7 +224,7 @@ void expand_message_xmd(uint8_t out[256], const void *msg, size_t msgSize, const
 	h.update(dst, dstSize);
 	h.digest(out, mdSize, &dstSizeBuf, 1);
 	uint8_t mdXor[mdSize];
-	for (size_t i = 1; i < ell; i++) {
+	for (size_t i = 1; i < n; i++) {
 		h.clear();
 		for (size_t j = 0; j < mdSize; j++) {
 			mdXor[j] = md[j] ^ out[mdSize * (i - 1) + j];
diff --git a/test/mapto_wb19_test.cpp b/test/mapto_wb19_test.cpp
index 3041419..3dbf8f9 100644
--- a/test/mapto_wb19_test.cpp
+++ b/test/mapto_wb19_test.cpp
@@ -890,7 +890,17 @@ void testHashToFp2v7(const T& mapto)
 		size_t msgSize = strlen(msg);
 		size_t dstSize = strlen(dst);
 		uint8_t md[256];
-		mcl::fp::expand_message_xmd(md, msg, msgSize, dst, dstSize);
+		mcl::fp::expand_message_xmd(md, sizeof(md), msg, msgSize, dst, dstSize);
+		CYBOZU_TEST_EQUAL(toHexStr(md, sizeof(md)), expect);
+	}
+	{
+		char msg[] = "asdf";
+		char dst[] = "QUUX-V01-CS02-with-BLS12381G1_XMD:SHA-256_SSWU_RO_";
+		char expect[] = "ecc25edef8f6b277e27a88cf5ca0cdd4c4a49e8ba273d6069a4f0c9db05d37b78e700a875f4bb5972bfce49a867172ec1cb8c5524b1853994bb8af52a8ad2338d2cf688cf788b732372c10013445cd2c16a08a462028ae8ffff3082c8e47e8437dee5a58801e03ee8320980ae7c071ab022473231789d543d56defe9ff53bdba";
+		size_t msgSize = strlen(msg);
+		size_t dstSize = strlen(dst);
+		uint8_t md[128];
+		mcl::fp::expand_message_xmd(md, sizeof(md), msg, msgSize, dst, dstSize);
 		CYBOZU_TEST_EQUAL(toHexStr(md, sizeof(md)), expect);
 	}
 	{
@@ -1054,6 +1064,12 @@ void testEth2phase0()
 	}
 }
 
+template<class T>
+void testHashToG1(const T& mapto)
+{
+	(void)mapto;
+}
+
 CYBOZU_TEST_AUTO(test)
 {
 	initPairing(mcl::BLS12_381);
@@ -1076,4 +1092,5 @@ CYBOZU_TEST_AUTO(test)
 	testHashToFp2v6(mapto);
 	testHashToFp2v7(mapto);
 	testEth2phase0();
+	testHashToG1(mapto);
 }