To exclude uselessness at fitting of the slider of a sliding door and an opening preventing mechanism.

In the guide device of the sliding door 40 constituted of a rail 35 fitted to the underside of the upper frame 3 of a frame body 30 fitting a sliding door 40, and a slider 1 fitted to the angle part crossing the upper face 40a of the sliding door 40 and a running direction face 40b and fitted with the rail 35, a magnet catch 20 is provided on the front face 1a of the slider 1 exposed in the running direction face 40b of the sliding door 40, and by the work of one time fitting the slider 1 to the sliding dopor 40, the magnet catch is also mounted on the sliding door 40.