An image forming apparatus includes a main casing, a plurality of photosensitive drums, a retaining unit, and an endless belt. The retaining unit retains the plurality of photosensitive drums, and is movable between an internal position within the main casing and an external position outside of the main casing. The endless belt is configured to contact the plurality of the photosensitive drums, when the retaining unit is in the internal position and retains the plurality of the photosensitive drums therein. The retaining unit includes a plurality of pressing portions configured to exert pressure on the plurality of photosensitive drums toward the endless belt, when the retaining unit is in the internal position and retains the plurality of the photosensitive drums therein.