To prevent erroneous operation of an occupant crash protection device by certainly detecting failure of an AD conversion part of an occupant detection device.

The AD conversion part 111 AD-converts an output signal of a load sensor 120, and an occupant detection control part 112 determines existence of an occupant and an object and a kind of the occupant based on the AD conversion output signal and outputs it as occupant detection data DT. An air bag deployment control part 211 deploys/controls an air bag body 230 based on the detection signals of an impact detection sensor 220 and a G sensor 214 and the occupant detection data DT. The occupant detection control part 112 determines it as failure of the AD conversion part 111 when the new AD conversion output signal is not obtained even if a predetermined time is passed after obtaining the AD conversion output signal, and outputs the failure signal to the air bag deployment control part 211. The air bag deployment control part 211 receiving the failure signal informs the occupant of the failure by a warning lamp.