An occupant detection system includes: a front weight sensor group formed from two weight sensors that are connected in series and arranged to the left and right on an upper face of a seat cushion; and a rear weight sensor group formed from two weight sensors that are connected in series and arranged to the left and right on the upper face of the seat cushion. The weight sensor groups are connected in parallel to each other. Regardless of whether an occupant sits on a front or rear part of a seat, since the load of the buttocks always acts on both left and right sides of the seat cushion, two series-connected left and right weight sensors of at least one of the plurality of weight sensor groups are both turned on without fail, thereby reliably detecting the occupant.