The invention provides a semiconductor device. Provided is an integrated circuit having a LOCOS-drain type MOS transistor mounted thereon in which, even in the case of poor pattern formation, a withstand voltage is not lowered and a poor withstand voltage does not result. A drain oxide film thicker than a gate oxide film is formed on an active region on a drain side of the LOCOS-drain type MOS transistor, to thereby prevent the withstand voltage of the MOS transistor from being lowered even if the gate electrode reaches the active region on the drain side.