An image forming apparatus to which a cartridge including an image bearing member and a frame supporting the image bearing member is detachably mountable includes a first supporting roller, a second supporting roller, a belt member for transferring a developed image formed on the image bearing member onto a recording material, and an opening to permit the cartridge to enter an inside of a main assembly of the apparatus. In addition, a first guide portion guides the cartridge through the opening from a position where at least a part of the image bearing member is in a back side, with respect to an extension plane, extending from a surface of the belt member disposed between the first and second rollers, and a second guide portion guides the cartridge to a position where the image bearing member is on a front side of the extension plane opposite to the back side and spaced from the extension plane. A third guide portion guides the cartridge from a position where the image bearing member is spaced from the extension plane in the front side of the extension plane to the transfer position.