A sliding member that is provided in a front panel of a case so as to be movable in the vertical direction and is placed at a down position due to the own weight is provided with a window portion which covers and hides a screw insertion hole for fixing a unit when the sliding member is located at the down position and exposes the screw insertion hole when the sliding member is lifted to an up position. Further, the front panel is provided with an interlocking switch that controls the operation of the case unit in a manner such that the interlocking switch is turned on while being pressed by the sliding member lifted to the up position and is turned off while the pressing operation is released when the sliding member is located at the down position.