A unit has a bottom surface, and first and second flange portions. A mounting plate has a first hook portion to be engaged with the first flange portion to control movement of the unit, having been mounted onto an upper plate surface, toward a front side in a depth direction and in a height direction; a second hook portion to be engaged with the second flange portion to control movement of the unit, having been mounted onto the upper plate surface, toward the front side in the depth direction and in the height direction; and a locking portion releasably engaged with the second flange portion to allow movement of the unit, being inserted or removed on the upper plate surface in the depth direction, and to control movement of the unit, having been mounted onto the upper plate surface, toward a rear side in the depth direction.