A partition member is disposed in a space portion between a rear surface of a housing and outer surfaces of a plurality of units. A first partition portion is disposed in a boundary portion between two units adjacent to each other in a vertical direction and extends in a right-left direction. A second partition portion and a third partition portion each are connected to a corresponding one of both ends of the first partition portion in the right-left direction and extend toward the upper surface. Air discharged from a first unit group located above the first partition portion is guided to flow through a first air channel to the upper surface. Air discharged from a second unit group located below the first partition portion is guided to flow through a second air channel to the upper surface.