The clutch includes a drive disk, a follower disk and a brake disk. When current flows, the follower disk is connected with the drive disk to transmit a power from a driving unit to a door-opening/closing mechanism. When no current flows, the follower disk is separated from the drive disk to cut off the power transmission from the driving unit to the door-opening/closing mechanism. In a state that the drive disk and the follower disk are separated from each other, the brake disk is connected to the follower disk to brake the door-opening/closing mechanism in the opening direction of the door, and to allow the door-opening/closing mechanism to move in the closing direction of the door.