OCCUPANT DETECTION SYSTEM AND OCCUPANT DETECTION DEVICE

An occupant detection system includes a transmitter provided adjacent to a first side face of a vehicle before a predetermined seat and successively transmitting a radio wave over a range covering a seat door located on a second side face of the vehicle and in a same row as the predetermined seat, a receiver provided adjacent to the second side face behind the predetermined seat and receiving the radio wave transmitted from the transmitter, and an occupant detection device including a reception intensity acquirer, an occupant detector, an opening and closing detector. The reception intensity acquirer successively acquires a reception intensity of a radio wave successively received by the receiver. The occupant detector detects an occupant in the vehicle according to the reception intensity of the radio wave received by the receiver while the opening and closing detector detects that the seat door is open.