There are provided a cam 23 which can be rotated by a releasing motor M4, a contact/separation lever 24 which is moved according to the rotation of the cam 23, a light shielding plate 25 and a cam position detecting sensor S7 which detect the rotational position of the cam, and a CPU circuit unit for controlling the rotation of the releasing motor based on the detecting operation by the light shielding plate and the cam position detecting sensor. The CPU circuit unit controls the rotation of the releasing motor M4 in such a manner that the cam is rotated reversely when a state detected by the light shielding plate and the cam position detecting sensor is not varied even if the releasing motor M4 is controlled to be rotated.