diff --git a/include/mcl/she.hpp b/include/mcl/she.hpp
index d441e1c..90db0f4 100644
--- a/include/mcl/she.hpp
+++ b/include/mcl/she.hpp
@@ -544,6 +544,7 @@ private:
 	struct ZkpBinTag;
 	struct ZkpEqTag; // d_[] = { c, sp, ss, sm }
 	struct ZkpBinEqTag; // d_[] = { d0, d1, sp0, sp1, ss, sp, sm }
+	struct ZkpDecTag; // d_[] = { c, h }
 public:
 	/*
 		Zkp for m = 0 or 1
@@ -557,6 +558,10 @@ public:
 		Zkp for (m = 0 or 1) and decG1(c1) == decG2(c2)
 	*/
 	typedef ZkpT<ZkpBinEqTag, 7> ZkpBinEq;
+	/*
+		Zkp for Dec(c) = m
+	*/
+	typedef ZkpT<ZkpDecTag, 2> ZkpDec;
 
 	typedef CipherTextAT<G1> CipherTextG1;
 	typedef CipherTextAT<G2> CipherTextG2;
@@ -775,6 +780,50 @@ public:
 				return isZero(c.a_);
 			}
 		}
+		int64_t decWithZkpDec(bool *pok, ZkpDec& zkp, const CipherTextG1& c, const PublicKey& pub) const
+		{
+			/*
+				c = (S, T)
+				S = mP + rxP
+				T = rP
+				R = S - xT = mP
+			*/
+			G1 R;
+			G1::mul(R, c.T_, x_);
+			G1::sub(R, c.S_, R);
+			int64_t m = PhashTbl_.log(R, pok);
+			if (!*pok) return 0;
+			const G1& P1 = P_;
+			const G1& P2 = c.T_; // rP
+			const G1& A1 = pub.xP_;
+			G1 A2;
+			G1::sub(A2, c.S_, R); // rxP
+			Fr b;
+			b.setRand();
+			G1 B1, B2;
+			G1::mul(B1, P1, b);
+			G1::mul(B2, P2, b);
+			char buf[sizeof(G1) * 5];
+			cybozu::MemoryOutputStream os(buf, sizeof(buf));
+			P2.save(os);
+			A1.save(os);
+			A2.save(os);
+			B1.save(os);
+			B2.save(os);
+			Fr& d = zkp.d_[0];
+			Fr& h = zkp.d_[1];
+			h.setHashOf(buf, os.getPos());
+			Fr::mul(d, h, x_);
+			d += b;
+			return m;
+		}
+		int64_t decWithZkpDec(ZkpDec& zkp, const CipherTextG1& c, const PublicKey& pub) const
+		{
+			bool b;
+			int64_t ret = decWithZkpDec(&b, zkp, c, pub);
+			if (!b) throw cybozu::Exception("she:SecretKey:decWithZkpDec");
+			return ret;
+		}
 		template<class InputStream>
 		void load(bool *pb, InputStream& is, int ioMode = IoSerialize)
 		{
@@ -1288,6 +1337,38 @@ public:
 			const MulG<G1> xPmul(xP_);
 			return verifyZkpBin(c.S_, c.T_, P_, zkp, PhashTbl_.getWM(), xPmul);
 		}
+		bool verify(const CipherTextG1& c, int64_t m, const ZkpDec& zkp) const
+		{
+			/*
+				Enc(m;r) - Enc(m;0) = (S, T) - (mP, 0) = (S - mP, T)
+			*/
+			const Fr& d = zkp.d_[0];
+			const Fr& h = zkp.d_[1];
+			const G1& P1 = P_;
+			const G1& P2 = c.T_; // rP
+			const G1& A1 = xP_;
+			G1 A2;
+			G1::mul(A2, P_, m);
+//			PhashTbl_.getWM().mul(A2, m);
+			G1::sub(A2, c.S_, A2); // S - mP = xrP
+			G1 B1, B2, T;
+			G1::mul(B1, P1, d);
+			G1::mul(B2, P2, d);
+			G1::mul(T, A1, h);
+			B1 -= T;
+			G1::mul(T, A2, h);
+			B2 -= T;
+			char buf[sizeof(G1) * 5];
+			cybozu::MemoryOutputStream os(buf, sizeof(buf));
+			P2.save(os);
+			A1.save(os);
+			A2.save(os);
+			B1.save(os);
+			B2.save(os);
+			Fr h2;
+			h2.setHashOf(buf, os.getPos());
+			return h == h2;
+		}
 		bool verify(const CipherTextG2& c, const ZkpBin& zkp) const
 		{
 			const MulG<G2> yQmul(yQ_);
@@ -1896,6 +1977,7 @@ typedef SHE::CipherText CipherText;
 typedef SHE::ZkpBin ZkpBin;
 typedef SHE::ZkpEq ZkpEq;
 typedef SHE::ZkpBinEq ZkpBinEq;
+typedef SHE::ZkpDec ZkpDec;
 
 inline void init(const mcl::CurveParam& cp = mcl::BN254, size_t hashSize = 1024, size_t tryNum = local::defaultTryNum)
 {
diff --git a/test/she_test.cpp b/test/she_test.cpp
index f2651d1..f7095ec 100644
--- a/test/she_test.cpp
+++ b/test/she_test.cpp
@@ -331,6 +331,25 @@ CYBOZU_TEST_AUTO(ZkpBinEq)
 	ZkpBinEqTest(sec, ppub);
 }
 
+CYBOZU_TEST_AUTO(ZkpDec)
+{
+	const SecretKey& sec = g_sec;
+	PublicKey pub;
+	sec.getPublicKey(pub);
+	CipherTextG1 c;
+	int m = 123;
+	pub.enc(c, m);
+	ZkpDec zkp;
+	CYBOZU_TEST_EQUAL(sec.decWithZkpDec(zkp, c, pub), m);
+	CYBOZU_TEST_ASSERT(pub.verify(c, m, zkp));
+	CYBOZU_TEST_ASSERT(!pub.verify(c, m + 1, zkp));
+	CipherTextG1 c2;
+	pub.enc(c2, m);
+	CYBOZU_TEST_ASSERT(!pub.verify(c2, m, zkp));
+	zkp.d_[0] += 1;
+	CYBOZU_TEST_ASSERT(!pub.verify(c, m, zkp));
+}
+
 CYBOZU_TEST_AUTO(add_sub_mul)
 {
 	const SecretKey& sec = g_sec;