The retaining unit is configured to retain a plurality of photosensitive drums juxtaposedly arrayed with each other in a predetermined direction. The retaining unit is movable between an internal position within the main casing and an external position outside of the main casing. Each photosensitive drum includes a first pressed portion and a second pressed portion opposite to the first pressed portion in an axial direction. The endless belt is disposed above the retaining unit that is in the internal position. The retaining unit includes a pair of side plates. Each of the pair of side plates includes a plurality of receiving portions each of which being configured to receive each photosensitive drum to be movable in a vertical direction. The first pressed portion and the second pressed portion protrude outward from a corresponding one of the plurality of receiving portions in the axial direction.