An occupant detection system is a part of a vehicle occupant protection system. The vehicle occupant protection system is enabled to be activated when the occupant detection system determines an ON state indicative of existence of an occupant on a seat. The vehicle occupant protection system is disabled to be activated when the occupant detection system determines an OFF state. The system includes a capacitive sensor which detects the occupant based on a capacitance around the seat. The system determines the ON state or the OFF state based on the detected value of the capacitive sensor and a predetermined threshold value. The system evaluates whether the determination result in the determination block is appropriate or not. Then, the system biases the threshold value to suppress a reversal determination from the determination result which is evaluated as appropriate.