An image forming apparatus includes a belt unit, a plurality of photosensitive drums, a cartridge, a casing, a cartridge holding unit and a first guide unit. The looped belt has a first surface outside of the loop, and is stretched to form a substantially straight portion on the loop that extends in a first direction. The casing has a base surface. The cartridge holding unit holds the cartridge at a first predetermined position in the casing. Each circumferential surface of the plurality of photosensitive drums contacts the straight portion of the first surface when the cartridge is held by the cartridge holding unit at the first predetermined position. The first guide unit guides the cartridge into the first predetermined position, and allows the plurality of photosensitive drums to move apart from the straight portion of the first surface when the cartridge is detached from the first predetermined position.