Forward tilting of a seat back and pushing of the seat forward allow fixation between an upper slide and an upper slide rail to be released, thereby allowing the upper slide to reach a tip end position of the upper slide rail. With this structure, a lower lock mechanism releases fixation of a lower slide to a lower slide rail and a vertical slide lock mechanism fixes the upper slide and the lower slide. Further, pushing the seat back toward a seat front side allows the lower slide integrally provided with the upper slide to move to a tip end position of the lower slide rail.