A vehicle seat for receiving an occupant is placed on a slider so as to move along a guide rail. A pair of belt anchor brackets for anchoring each end of a lap belt at each side of the seat is provided under the guide rail, and a weight sensor for sensing weight variation of the seat is provided under the belt anchor bracket. The tension of a seat belt (lap belt) does not affect the weight sensing functionality of the weight sensor and the state of the seat can accurately be determined.