PURPOSE: To improve dust collection effect and to prolong the life of a rilter, etc., by switching the air blasting means of a dust between suction and blasting corresponding to picture formation processes.

CONSTITUTION: An electromagnetic device 35 operates a photosensitive drum 1 and a suction blower 27 simultaneously and a guide 25b is held through a plunger 36 and a spring 34 on the side where an opening part 25a is formed, so a developer scattered from a developing device 3 passes through a dust collecting duct 25 from the opening part 25a as shown by an arrow to be collected in a dust collecting filter 26. When copying operation ends and a sleeve 14, the photosensitive device 1, and electromagnetic device 35 get to inoperative successively, the guide 25b blocks the opening part 25a extending to the suction duct 25 and only a blast from a blowoff duct 29 operates on the opening part of the developing device to functions as an air curtain at the opening part, so the developer is prevented from scattering from the opening part.