To facilitate a posture change to a developed posture or a stored posture by operation from a swing end side even if a supplementary top plate having a large swing radius is used when a supporting device of a slide type which is advantageous in terms of strength.

This plate-shaped supporting device swings relatively around a swing fulcrum P1 almost along the horizontal direction for a member 1 to be attached and is constructed so that the attaching angle can be changed to a developed posture and a stored posture. In this case, it is constructed to switch a supporting state in which a part of an extended supporting member 7 of a supporting metal fitting 3 fixedly attached to the member 1 so as to support the load of a developing-postured plate-shaped member 2 is projected downward from the developing-postured plate-shaped member 2 below the fulcrum P1 of the plate-shaped member 2, and a support cancelled state in which the part of the extended supporting member 7 is pulled back toward the member 1 side from the fulcrum 1. A switching operation part 5 for switching the postures of the extended supporting member 7 is constructed so as to swing as well as the plate-shaped member 2. There is provided a means by which the posture switching operation can be performed at a position closer to the swinging end of the plate- shaped member 2 than the extension supporting member 7.