A cartridge, which is an attachment device, is mountable into an insertion slot 8 in a game apparatus. At a side surface inside the insertion slot 8, provided is a slide member 41 movable from a near side to a far side of the insertion slot 8. The attachment device includes a housing 11 and a hook 14. When the attachment device is inserted into the insertion slot 8, a predetermined portion of the housing 11 abuts against the slide member 41, and the slide member 41 is caused to move toward the far side of the insertion slot 8 when the predetermined portion is inserted further inside while abutting against the slide member 41. The hook 14 is protrusible from a side surface of the housing 11, and is inserted into a recessed portion (an air gap A1) generated at the side surface inside the insertion slot 8 when the slide member 41 moves toward the far side of the insertion slot 8, and then latches onto the inside wall.