A passenger seat device includes a detector electrode for detecting occupancy of the seat, a seat heater powered by an on-board electric power source, and a waterproof sheet for preventing water permeated into the seat from reaching the seat heater. Whether the seat is occupied or not is detected based on changes in an impedance coupled to the detector electrode. The impedance is affected by the water if it reaches the seat heater, and a sensitivity of the occupancy detection is adversely affected. A shielding electrode insulated from the detector electrode and disposed between the detector electrode and the seat heater may be provided to reduce an amount of parasitic capacitance formed between the detector electrode and the seat heater. An alternating voltage may be supplied to the detector electrode and the shielding electrode.