To provide an occupant detection system, capable of detecting existence of sitting of the occupant stably even when disturbances, such as water-pouring sheet of a vehicle, are present, and to provide a control method of the system.

The occupant detection system includes an electrode 75 provided in a vehicle seat for detecting the capacitance C1 derived from a human body, an oscillator circuit 41 that supplies a reference signal S0 containing sinusoidal waves to the electrode for detecting via a resistive element, a first comparator circuit 43 that binarizes the reference signal S0 using a threshold corresponding to a given phase so as to produce a reference phase signal D0, a second comparator circuit 44 that detects the potential of the electrode for detecting as an electrode signal S1, and binarizes the electrode signal S1 using a threshold corresponding to a phase that is substantially the same as a given phase of the reference signal S0 so as to produce an electrode phase signal D1, and a control circuit 6 that measures delay times of the electrode phase signal D1 relative to a rise timing and a down timing of the reference phase signal D0, so as to determine the presence of a seated occupant based on the delay times.