A capacitance type sensor installed in a vehicle includes the first electrode, the second electrode, and the third electrode. The first electrode is arranged inside a seating portion of a seat or a backrest portion of the seat, or both. The second electrode is arranged adjacent to the first electrode inside at least one of the seating portion and the backrest portion of the seat, whatever the first electrode is arranged. The second electrode forms an electric field together with the first electrode between them in a wetness determination mode in which a wet condition of the seat is determined. The third electrode forms an electric field together with the first and the second electrode between them in an occupant determination mode in which an occupant in the seat is determined.