An image forming apparatus includes an image forming unit, a toner container mount, first and second toner containers removably insertable into the toner container mount horizontally, container guides formed in toner container mount to guide lateral side portions of the first and second toner containers, respectively, and an insertion opening forming member positioned defining first and second insertion openings through which the first and second toner containers are inserted, each including a laterally projecting portion. The second toner container is greater than the first toner container in cross section perpendicular to a direction of insertion, and the second insertion opening is greater than the first insertion opening. The first and second toner containers include guided portions projecting laterally, and the guided portion of the first toner container has a vertical length longer than that of the guided portion of the second toner container.