Electrostatic occupant detection system

An electrostatic occupant detection system (10, 10a-10d) includes an electrostatic sensor (31) and an electronic control unit (11, 11 a-11 d). The electronic control unit (11, 11 a-11 d) is switchable between an occupant determination state in which the electronic control unit (11, 11a-11d) outputs a sine wave having a constant amplitude and a diagnosis state in which the electronic control unit (11, 11a-11d) maintains a voltage of the electrostatic sensor (31) at a constant level. The electronic control unit (11, 11a-11d) gradually changes at least one of an amplitude and a frequency of the sine wave either when the electronic control unit (11, 11a-11d) switches from the occupant determination state to the diagnosis state and/or when the electronic control unit (11, 11a-11d) switches from the diagnosis state to the occupant determination state.