A stopper (31) has an installation section (32) installed on a bottom wall (12) of a lower rail (3) while in contact with an outer surface (12a) of the bottom wall (12), and also has a pair of flanges (33) extending from the installation section (32) so as to pass through an insertion hole (30) in the bottom wall (12) and placed in an internal space of the lower rail (3). The flanges (33) engage with second cut and raised sections (23b), provided on an upper rail (4), to limit the range of movement of the upper rail (4) relative to the lower rail (3).