To reduce an abutment sound produced when an output rotary part of a one-way power transmission mechanism is released from a self-lock state in releasing the parking brake.

An one-way power transmission mechanism C provided to a rotating drive transmission system between an electric motor and a conversion mechanism is equipped with: an input rotary part 23a rotatably driven by the electric motor; an output rotary part 41a which is coaxially coupled to the input rotary part 23a and which is relatively rotatable by a predetermined quantity, a non-rotatable fixing tube part 33a which surrounds the outer peripheries of coupling parts of both rotary parts 23a, 41a; a clutch coil spring 24 accommodated in the fixing tube part 33a so as to be slidable circumferentially; and a torque transmitting member 25 provided between the input rotary part 23a and output rotary part 41a in the clutch coil spring 24. The torque transmitting member 25 transmits a reverse rotation torque to the output rotary part 41a from the input rotary part 23a, before the input rotary part 23a is driven to reversely rotate and engaged with the output rotary part 41a through a release side end 24b of the clutch coil spring 24.

COPYRIGHT: (C)2008,JPO&INPIT