A disk brake with a parking brake mechanism capable of exerting a large piston thrust required for operating a parking brake, without adversely affecting operation of a service brake. A parking brake mechanism 32, which is driven by an electric motor provided outside a housing 30, is incorporated in a caliper 14 in which a piston 26 is slidably disposed in a cylinder 24. The parking brake mechanism 32 is slidably fitted via a seal member 36 into the piston 26 and provided with a nut member 37 that is prevented from rotating relative to the piston 26 by engagement of a pin 48 and a pin hole 49; and a shaft 35 that is screwed into the nut member 37. During a service brake operation, the piston 26 alone is moved by a hydraulic pressure under a small piston thrust. During a parking brake operation, the piston 26 and the nut member 37 are moved together by applying a hydraulic pressure and operating the electric motor at the same time, to exert a large piston thrust, by using a large pressure receiving area of the piston 26 and the nut member 37 combined.