A sliding device for moving a seat in the lengthwise direction of a vehicle body is comprised of an upper rail having an inner space and supporting the seat, a lower rail having an inner space, secured to a floor of the vehicle floor and mounting the upper rail in such a manner that the upper and lower rails are inter-locked with each other, the upper rail is slidable along the lower rail and the inner space of the upper rail is positioned above the inter-locked portion, a screw shaft rotatably supported by the upper rail and located therein, a nut threadably mounted on the screw shaft and secured to the lower rail so as to be located in the inner space of the upper rail, and a motor for rotating the screw shaft.