To prevent a front link from deforming and falling down on the action of a submarine load to a horizontal bar without complicating the configuration.

The right and left front links 12 (L, R) which are pivotally mounted to a seat riser 24 at one end are formed integrally having a reinforcing flange 30 extending to the inner side of the seat by a prescribed length on at least either the front end side or the rear end side. The right and left front links and the horizontal bar are formed as an integral member of truss structure by an integral fixture of the horizontal bar 14 on the submarine track of a seated person's hips for at least the reinforcing flange of the right and left front links.