An electrostatic seat occupant detecting system of a vehicle has a vehicle body and a seat. The electrostatic seat occupant detecting system includes an antenna unit that includes a main electrode and a sub-electrode disposed near the main electrode, an AC power source, a switch unit for switching supply of AC voltage to the antenna unit to form one of a first electric field between the vehicle body and the main electrode thereby providing a seat-occupant detecting mode and a second electric field between the main electrode and the sub-electrode thereby providing a wet condition detecting mode, and a seat-occupant detecting ECU. The above seat-occupant detecting ECU includes an admittance calculating section that calculates a first admittance of the first electric field and a second admittance of the second electric field, a conductance susceptance separating section that separates conductance of the first and second admittance from the susceptance of the first and second admittances, a water content estimating section that estimates an amount of water content of the seat based on the conductance and the susceptance of the second admittance, and a judging section that judges whether a seat occupant is present or not based on one of the conductance and susceptance of the first admittance and the estimated amount of the water content of the seat.