A lower rail or an upper rail has a first wall provided at a position on a first one of opposite sides in a widthwise direction thereof, and a second wall provided at a position on the other, second, side in the widthwise direction and disposed opposed to the first wall. At least one of the first wall and the second wall has an opening. At least one of an outer portion of the holding member in the widthwise direction and an outer portion of the nut member in the widthwise direction is inserted in the opening and disposed opposed to an inner side-surface of the opening facing in a frontward-rearward direction.