A power transmission mechanism includes a cam-incorporated gear which rotates in one rotational direction by a driving force transmitted from a drive source via a planet gear when engaged with the planet gear, wherein the cam-incorporated gear stops rotating when disengaged from the planet gear; a driven member which includes a follower which contacts at least one surface cam formed on the cam-incorporated gear, the driven member being driven in accordance with the rotation of the cam-incorporated gear in the one rotational direction, and exerting a surplus torque on the cam-incorporated gear in the one rotational direction thereof so as to increase the rotation of the cam-incorporated gear in the one rotational direction, the surplus torque being independent of a torque exerted on the cam-incorporated gear by the drive source; and a damping mechanism which continuously exerts a load torque on the cam-incorporated gear in a rotational direction opposite to the one rotational direction of the cam-incorporated gear so as to cancel out the surplus torque.