A tray unit is mounted on a rail adjacent to a bed and includes a motor driven tray that can move along the rail. When the pressure of the cooling air is high, the piston is forced out of the path of the cooling air and into an open position. The first member has a first attachment member that has side walls, a spine that connects the side walls, and a button unit that has a push button which is urged to the outside from the inside of the coupled members in the direction intersecting the sliding direction of the coupled members.