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