An occupant detection system including an electrostatic sensor, a determination section and a heater is disclosed. The electrostatic sensor includes an electrode arranged inside a seat of a vehicle and configured to generate a weak electric field. The electrostatic sensor outputs an output value associated with the weak electric field. The determination section supplies voltage to the electrode, and performs an occupant determination to determine an occupant on the seat based on the output value of the electrostatic sensor. The heater is inside the seat, and is arranged in parallel with or on a lower side of the electrode. Before performing the occupant determination, the determinations section determines whether or not the heater has a disconnection fault based on the output value of the electrostatic sensor.