Occupant detection system

An occupant detection system includes: a front weight sensor group (13) formed from two weight sensors (13L,13R) 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 (14) formed from two weight sensors (14L,14R) that are connected in series and arranged to the left and right on the upper face of the seat cushion (11). The weight sensor groups (13,14) are connected in parallel to each other.