To achieve a quick and easy moving operation of a mobile top board while letting it move to a position that allows the switching of the mobile top board to a housing attitude keeping a support leg at an attitude along the length of the side rim of a plate part.

A link device 5 is connected free to oscillate relatively about a vertical axis in such a manner as to be freely switchable to a position at which a mobile top board 3 is linearly extended in the direction orthogonal to the longitudinal direction of a side rim of a plate part 2 and to a position at which it is folded at an attitude in the longitudinal direction of the side rim of the plate part 2 over a support leg 4 placed and supported at an expanded attitude and the body 1 of furniture. A guide part is disposed to guide and move the mobile top board 3 to a position that allows the switching of the mobile top board to a housing attitude while the support leg 4 is kept at an attitude along the longitudinal direction of the side rim of the plate part 2 following a folding operation of the link device 5. To build the guide part, a guide groove 12 is formed at least on the rear of the mobile top board 3 along the direction orthogonal to the longitudinal direction of the side rim of the plate part 2 while a plurality of guide rollers 13 rolling within the guide groove 12 are arranged at an upper part of the support leg 4 along the length of the guide groove 12.