A seat slide device is fixed to the floor of a vehicle and includes lower rails, each of which includes a front range and a rear range and extends forward and backward, and an upper rail supporting a movable seat and capable of moving over the lower rails. The seat slide device also includes a lock portion, a stopper, and a stopper-releasing operating portion. The lock portion selectively restricts movement of the upper rail on the lower rails at a plurality of positions. The lock portion is provided to the upper rail. The stopper stops movement of the upper rail before the upper rail advances from either one of the front range and the rear range, past this range, to the other range. The stopper-releasing operating portion enables movement of the upper rail when the upper rail has been stopped by the stopper.