PURPOSE: To prevent an offset securely by providing a transfer drum so that it can leave a photosensitive drum, and allowing the transfer drum to leave the photosensitive drum during nontransfer peripheral motion of a toner developed image.

CONSTITUTION: Oscillatory plates 44 and 45 which supports the shaft 43 of the transfer drum 33 are pivoted in an oscillatory state on a shaft 46 eccentric with the shaft 43. Cam followers 49 and 50 are provided to the shafts 47 and 48 of the oscillatory plates 44 and 45, and cams 52 and 53 are fitted to a cam shaft 51 opposite to the cam followers. In this constitution, a driving gear 55 is driven during the rotation of a photosensitive drum 1 which does not contribute to transfer, and the cams 52 and 53 each makes a half turn through an electromagnetic clutch 54 to turn the oscillatory plates 44 and 45 by the cam followers 49 and 50, so that the transfer drum 33 leaves the photosensitive drum 1. Consequently, such an offset that toner after transfer sticks to the photosensitive drum 1 is prevented securely.