An image forming apparatus includes a plurality of image forming units, each provided with an image bearing member, detachably mountable to a main assembly of the image forming apparatus; an opening, provided in a side surface of the main assembly, for permitting insertion of the plurality of image forming units into the opening; a guide portion for guiding the plurality of image forming units toward the opening; and a projected portion, provided on the guide portion, for positioning a part of an associated image forming unit outside the opening by contacting the associated image forming unit when the associated image forming unit is inserted into the opening with an attitude different from the attitude of the associated image forming unit at the time when the associated image forming unit is disposed in the main assembly.