A sensor body is interposed between a leg bracket and a lower rail and detects a seat load. A plate member of the sensor body is a unit including a circular strain element having an axial line in the vertical direction and a substrate extending radially from the strain element. An outer circumferential support member is attached to the leg bracket to hold the outer circumferential edge of the strain element. An inner circumferential support member is attached to the lower rail to hold the inner circumferential edge of the strain element. The strain element includes a strain gauge configured to output a signal in accordance with the load in the axial direction of the strain element on the basis of the distortion between the outer and inner circumferences. The substrate includes an electric circuit electrically connected the strain gauge and configured to process the output signal from strain gauge.