diff --git a/include/mcl/she.h b/include/mcl/she.h
index 093c7a0..20b88c4 100644
--- a/include/mcl/she.h
+++ b/include/mcl/she.h
@@ -119,6 +119,13 @@ MCLSHE_DLL_API int sheDecG1(int64_t *m, const sheSecretKey *sec, const sheCipher
 MCLSHE_DLL_API int sheDecG2(int64_t *m, const sheSecretKey *sec, const sheCipherTextG2 *c);
 MCLSHE_DLL_API int sheDecGT(int64_t *m, const sheSecretKey *sec, const sheCipherTextGT *c);
 
+/*
+	return 1 if dec(c) == 0
+*/
+MCLSHE_DLL_API int sheIsZeroG1(const sheSecretKey *sec, const sheCipherTextG1 *c);
+MCLSHE_DLL_API int sheIsZeroG2(const sheSecretKey *sec, const sheCipherTextG2 *c);
+MCLSHE_DLL_API int sheIsZeroGT(const sheSecretKey *sec, const sheCipherTextGT *c);
+
 // return 0 if success
 // z = x + y
 MCLSHE_DLL_API int sheAddG1(sheCipherTextG1 *z, const sheCipherTextG1 *x, const sheCipherTextG1 *y);
diff --git a/src/she_c_impl.hpp b/src/she_c_impl.hpp
index f7ea627..1eb676f 100644
--- a/src/she_c_impl.hpp
+++ b/src/she_c_impl.hpp
@@ -274,6 +274,30 @@ int sheDecGT(int64_t *m, const sheSecretKey *sec, const sheCipherTextGT *c)
 	return decT(m, sec, c);
 }
 
+template<class CT>
+int isZeroT(const sheSecretKey *sec, const CT *c)
+	try
+{
+	return cast(sec)->isZero(*cast(c));
+} catch (std::exception& e) {
+	fprintf(stderr, "err %s\n", e.what());
+	return 0;
+}
+
+int sheIsZeroG1(const sheSecretKey *sec, const sheCipherTextG1 *c)
+{
+	return isZeroT(sec, c);
+}
+int sheIsZeroG2(const sheSecretKey *sec, const sheCipherTextG2 *c)
+{
+	return isZeroT(sec, c);
+}
+int sheIsZeroGT(const sheSecretKey *sec, const sheCipherTextGT *c)
+{
+	return isZeroT(sec, c);
+}
+
+
 template<class CT>
 int addT(CT& z, const CT& x, const CT& y)
 	try
diff --git a/test/she_c_test.hpp b/test/she_c_test.hpp
index 84b171e..8a8a2a0 100644
--- a/test/she_c_test.hpp
+++ b/test/she_c_test.hpp
@@ -30,15 +30,30 @@ CYBOZU_TEST_AUTO(encDec)
 
 	int64_t m = 123;
 	sheCipherTextG1 c1;
+	sheCipherTextG2 c2;
 	sheCipherTextGT ct;
 	sheEncG1(&c1, &pub, m);
+	sheEncG2(&c2, &pub, m);
 	sheEncGT(&ct, &pub, m);
 
 	int64_t dec;
 	CYBOZU_TEST_EQUAL(sheDecG1(&dec, &sec, &c1), 0);
 	CYBOZU_TEST_EQUAL(dec, m);
+	dec = 0;
+	CYBOZU_TEST_EQUAL(sheDecG2(&dec, &sec, &c2), 0);
+	CYBOZU_TEST_EQUAL(dec, m);
+	dec = 0;
 	CYBOZU_TEST_EQUAL(sheDecGT(&dec, &sec, &ct), 0);
 	CYBOZU_TEST_EQUAL(dec, m);
+
+	for (int m = -3; m < 3; m++) {
+		sheEncG1(&c1, &pub, m);
+		CYBOZU_TEST_EQUAL(sheIsZeroG1(&sec, &c1), m == 0);
+		sheEncG2(&c2, &pub, m);
+		CYBOZU_TEST_EQUAL(sheIsZeroG2(&sec, &c2), m == 0);
+		sheEncGT(&ct, &pub, m);
+		CYBOZU_TEST_EQUAL(sheIsZeroGT(&sec, &ct), m == 0);
+	}
 }
 
 CYBOZU_TEST_AUTO(addMul)