A torque transmission mechanism includes a cam member which is rotatably supported and is provided with a cam for pressing a joint member in an axial direction of a rotation shaft and bush which is fitted to the cam member. The bush has an engagement surface with which the cam of the cam member engages. Above mechanism rotates the cam member and moves the cam member along the bush, thereby moving the joint member between an advanced position in which the torque of a drive source is transmitted to a rotary member and a retracted position in which the transmission of the torque of the drive source to the rotary member is released. A rib to which the bush is fitted is formed on an end surface of the cam member existing at a side at which the cam member is fitted to the bush.