A seat (70) moves along a rail (20) in the front-rear direction of a vehicle. A juxtaposed member (receiving member (51)) extending in the front-rear direction of the vehicle is joined to a floor (11) or the seat (70) so as to be located next to the rail (20). A section (59) to be detected is provided either to the juxtaposed member or to a member which is located on the floor (11) side or the seat (70) side and which moves relative to the floor (11) or the seat (70) to which the juxtaposed member is affixed. A sensor (60) for detecting the section (59) to be detected is provided either to the juxtaposed member or to the member located on the floor (11) side or the seat (70) side. As a result of this configuration, the mounting position of the sensor (60) is less spatially restricted than the case in which the sensor 60 is provided to the rail (20).
