A capacitive occupant detection system comprises a capacitive sensor and a seat frame located under the capacitive sensor through a seat cushion. The seat frame is electrically connected with the ground potential level of the vehicle. A water proof layer is placed above the seat frame. The water proof layer prevents water from penetrating the seat cushion to reach to the seat frame, even if water poured on the seating part. The water proof layer suppresses resistance decrease of resistive component between the capacitive sensor and the seat frame resulting from water poured on the seat.