diff --git a/gen/gen_code.cpp b/gen/gen_code.cpp
index d6f5e2a..b3fb123 100644
--- a/gen/gen_code.cpp
+++ b/gen/gen_code.cpp
@@ -441,6 +441,7 @@ void put()
 		} tbl[] = {
 			// only 64-bit mode(from)
 			{ "cdqe", B01001000, B10011000 },
+			{ "cqo", 0x48, 0x99 },
 			{ "@@@" }, /// here
 			// only 32-bit mode(from)
 			{ "aaa", B00110111 },
diff --git a/xbyak/xbyak_mnemonic.h b/xbyak/xbyak_mnemonic.h
index 9bcb79c..790385d 100644
--- a/xbyak/xbyak_mnemonic.h
+++ b/xbyak/xbyak_mnemonic.h
@@ -274,6 +274,7 @@ void jg(const char *label, LabelType type = T_AUTO) { opJmp(label, type, 0x7F, 0
 void setg(const Operand& op) { opR_ModM(op, 8, 0, 0x0F, B10010000 | 15); }
 #ifdef XBYAK64
 void cdqe() { db(0x48); db(0x98); }
+void cqo() { db(0x48); db(0x99); }
 #else
 void aaa() { db(0x37); }
 void aad() { db(0xD5); db(0x0A); }