A long aperture extending along the forward and backward movement direction is formed on each of the brackets, each of the supporting shafts is movably inserted into each of the long apertures, a spring shaft in parallel with one of the supporting shafts is protruded to the back of the front panel, a hold-down spring is formed in the door spring, the door spring is wound up by one of the supporting shafts, the free end of which is engaged with the tray door and also the hold-down spring is wound up by the spring shaft, the free end of which is engaged with the front panel, the forward-advanced tray pushes the tray door against the hold-down spring, by which the tray door is moved forward, and then the tray door is axially opened and moved around the supporting shaft against the door spring.