PURPOSE: To attain high accuracy of picture element positioning by providing a rotary encoder incorporatedly with a follower means.

CONSTITUTION: A gear 16 is provided incorporatedly onto the shaft of a drum 11 as a drive means rotated incorporatedly with a rotary drum 11. Further, a gear 17 is provided freely rotatably as a follower means to a frame 15 meshing directly with the gear 16. The number of teeth of the gear is selected less than those of the gear 16. The gear 17 is provided with a flange 20, and a pulse generating disc 22 constituting a rotary encoder 21 is fitted to the flange 20 and a photoelectric converting pulse generating circuit 23 of the rotary encoder 21 is fitted to the frame 15. Thus, highly accurate picture element position is attained by using the rotary encoder with small size and simple constitution in this way.