A lower rail is fixed to a vehicle floor, and an upper rail is movable in a rear and front direction with respect to the lower rail. A load sensor is fixed to an upper surface of the upper rail. A rectangular frame is attached onto the load sensor, a rod of the load sensor sequentially penetrates a web of the rectangular frame and a spring holder upward, and a coil spring is wound around the rod. A nut is screwed to the rod, and the nut tightens a bottom of a cup portion of the spring holder. The coil spring is sandwiched between a flange of the spring holder and the web and is compressed by tightening the nut.