A parking brake system is provided in which a parking brake state of a wheel brake is obtained by forward movement of a parking piston slidably fitted into a casing with the rear side of the parking piston facing a parking control fluid pressure chamber, a lock piston forming part of a lock mechanism for mechanically locking the forwardly moved state of the parking piston is slidably fitted into the casing to the rear of the parking piston and urged forwardly by a spring, an opening provide in a portion of the casing that the rear side of the lock piston faces is closed by a detachable lid member, a tool connection part enables a tool inserted through the opening to be detachably connected to the tool connection part. This enables an automatic parking brake state to be obtained by a simple structure without consuming power, and the parking brake state to be released by a manual operation for maintenance or inspection.