a piston;

a motor;

a rotary-linear motion converting mechanism for enabling rotation of a rotor of the motor to be converted to a linear motion and transmitted to the piston;

a caliper for pressing a brake pad against a disk rotor and thus generating a braking force when a thrust of the piston is generated according to the rotation of the rotor of the motor;

a park braking lock mechanism including a ratchet wheel and an engaging pawl, the ratchet wheel being adapted to be driven by the rotor of the motor, the park braking lock mechanism moving the engaging pawl to at least one of an engaged position relative to the ratchet wheel and a disengaged position relative to the ratchet wheel to effect a locking action and an unlocking action; and

a controller adapted to effect the locking action or the unlocking action of the park braking lock mechanism according to movement of the engaging pawl,

the caliper including displacement detection means adapted to detect displacement of the rotor of the motor,

the controller including unlocking failure detection means adapted to detect an unlocking failure of the park braking lock mechanism, based on results of a detection made by the displacement detection means,

wherein the unlocking failure detection means is operable to determine the occurrence of an unlocking failure of the park braking lock mechanism, but not an occurrence of a failure of the motor itself for applying the braking force, when the displacement detection means does not detect a displacement of the rotor of the motor while the controller is put in operation such that the engaging pawl moves to the disengaged position relative to the ratchet wheel to effect the unlocking action.