A tray for mounting a cartridge incorporating a flat information recording medium includes sliders (2,6) sliding in a loading direction of the cartridge on a tray base in cooperation with the mounting action of the cartridge, and a slider moving mechanism for moving the slider between an initial position, and a plurality of stopping positions remote from the initial position by specific distances in the loading direction. The plurality of stopping positions are at least in two stopping positions, that is, a first stopping position remote from the initial position by a first distance in the loading direction, and a second stopping position remote from the initial position by a second distance longer than the first distance in the loading direction. Thereby, if a cartridge is mounted on a tray in a wrong direction, the user can easily recognize its wrong mounting.