An image forming apparatus is so arranged as to form an image of an original document on a photosensitive member by accommodating the plate-like photosensitive member of a rectangular or square configuration placed on an exposure section base in a housing of the image forming apparatus. The exposure section base is disengageable from the housing in a horizontal direction and has a plurality of suction holes in a region on which the photosensitive member is placed. A switch detects when the exposure section base is inserted into the housing to reach a predetermined position, and a suction device then sucks air through the suction holes.