PURPOSE: To form a printed image of high quality by eliminating deviation in position of a blank lamp unit due to oscillation, etc., and to facilitate adjustment of the back and front positions.

CONSTITUTION: An adjustment knob 13 is fixed by being engaged to a triangular claw 17 on an edge surface and a rotation stopping claw 15 fixed to a processing frame part 14 and always being pressurized against the processing frame part 14 by a compressed spring provided inside the adjustment knob 13. A projection part 20 of a blank lamp holder 19 restricted to be moved only in directions of arrows (a) or (b) is fitted to an inclined groove 18 of the adjustment knob 13 and the blank lamp unit 5 is moved in the directions of the arrows (a) and (b) according to the number of pitches by rotating the adjustment knob 13 only for the number of pitches of the triangular claw 17.