A parking brake system is provided in which a parking brake state is obtained by forward movement of a parking piston in response to the action of a parking control fluid pressure, and a forward operating state of the parking piston is mechanically locked by a lock mechanism. Moreover, an input member of a brake mechanism and the parking piston are connected by a coupling linkage. A lock piston of the lock mechanism moves toward one side in the axial direction when the parking piston moves forward, and a locking part provided at one end of the lock piston engages from the rear with a retaining part provided on a middle section of the coupling linkage while straddling the middle section of the coupling. This enables an automatic parking brake state to be obtained by a simple structure without consuming power.