PURPOSE: To restrain the abrasion of a photosensitive drum and to obtain stable and excellent developing performance by changing the contact surface of the photosensitive drum with a tracking roll as the tracking roll is turned.

CONSTITUTION: The shape of the tracking roll 100 is set so that the position of the contact surface of the photosensitive drum 51 with the tracking roll may be changed as the tracking roll 100 is rotated. Namely, the tracking roll 100 abuts on the photosensitive drum 51 and is turned by interlocking with the turning of the photosensitive drum 51. Thus, the abrasion of the contact surface of the photosensitive drum 51 with the tracking roll is restrained and the stable and excellent developing performance is obtained.