Abstract: Only a left lock mechanism is coupled directly to an operation bar of comparatively high rigidity and a right lock mechanism is coupled to the left lock mechanism via a wire as a coupling member. The operation bar is rotatably supported at left and right end parts in left and right upper rails. The right end part is rotatably supported in a support pin of a bracket attached to the right upper rail slidably from side to side. A guide hole fitted slidably to an end part of an operation bar is made in a link for coupling the left lock mechanism and the end of the operation bar. A ring bushing made of a resin as a buffer member is fitted into the guide hole and is formed with an opposed face part opposed to the end part of the operation bar and an elastic piece pressed against the upper face of the operation bar.