PURPOSE: To regulate easily and accurately a mounting position of a LED head against a photosensitive body.

CONSTITUTION: An eccentric cam 14 is set on a shaft 16 which is integral with a head cover 4 of a LED head 2, and two faces of both sides of the cam 14 are made to contact and fixed to stoppers 18 and 20. Besides, cam faces 14-1-14-8 change a distance from a center of the cam 14 and make a distance between the cam face and the center so as to be changed, and make a distance between the cam face and the center change at every 1/8 rotation of the cam 14. A cam face by which a most adequate position against a photosensitive body 01 can be obtained is made to execute a positioning by executing a contact with a positioning member 02 of a side of the photosensitive body 01, and is used with a prevention of a rotation of the cam 14 by stoppers 18 and 20 after the positioning.