A seat moves along a rail in the front-back direction of a vehicle. A juxtaposed member (receiving member) extending in the front-back direction of the vehicle is joined to a floor or the seat so as to be located next to the rail. A detected portion is provided either to the juxtaposed member or to a member which is located on the floor side or the seat side which moves relative to the floor or the seat to which the juxtaposed member is coupled. A sensor that detects the detected portion is provided either to the juxtaposed member or to the member located on the floor side or the seat side. As a result of this configuration, the installation position of the sensor is less spatially restricted than the case in which the sensor is provided to the rail.