against the ends of the cutout portion 21 a of the lower rail 21 when the rail plate member 31 is inserted through the insertion slit 23 from above the step member 3 the press fit portions 31 b are pressed against the ends of the cutout portion 21 a of the lower rail 21 at this time the sides of the cutout portion 21 a may be slightly shaven or elastically deformed by the press fit portions 31 b in the above described configuration the rail plate member 31 can be removed to insert rollers 32 into the space between the lower rails 21 22 through the cutout portion 21 a or remove the rollers 32 from the space between the lower rails 21 22 when the drive device 2 is operated the belt 13 is rotated accordingly the rollers 32 are moved while being guided by the lower rails 21 22 and the sliding door is operated to open or close the output shaft 2 d of the drive device 2 or the housing of the drive device 2 has an seal ring which is not shown so that water is completely or almost completely prevented from entering the interior of.