In an image forming apparatus, a tray unit to which a cartridge is detachably mounted, and that is configured to be capable of moving between an internal position situated within the apparatus main body and an external position situated outside, includes first and second side plates configured to support the cartridge, and they are each made of metal, and include a supporting portion and an outer side portion, the supporting portions includes positioning portions configured to come into contact with the cartridge to position the cartridge as to an image forming position. The apparatus main body includes first and second pressing members each capable of moving between pressing positions at which the cartridge is pressed, and separated positions separated from the cartridge. The first and second pressing members are disposed each intersecting with imaginary planes that is orthogonal to the longitudinal direction and intersect the positioning portions.