A first retaining member is configured to be moved in a prescribed direction between a mounted position and a pulled-out position. A plurality of exposure units is configured to move between a first adjacent position in which each of the plurality of exposure units is located adjacent to the corresponding photosensitive drum to expose thereof and a first retracted position in which each of the plurality of exposure unit is moved away from the corresponding photosensitive drum in a direction intersecting with the prescribed direction. A space is formed by removing a tray from a main casing. Each of the plurality of exposure units is moved to the space to be moved to the first retracted position such that each of the plurality of exposure units fails to interfere with the first retaining member in the prescribed direction when the first retaining member moves from the mounted position to the pulled-out position.