PURPOSE: To provide an occupant protecting device and a judging device therefor that can diagnose the function of a safety sensor after mounting it to a vehicle.

CONSTITUTION: An air bag ECU 10 accumulatively integrating detected acceleration when the detected acceleration from an acceleration sensor 32 reaches the specified value. When the integrated value is the high value corresponding to the emergency state of a vehicle and safety sensor 17 is in the 'on' state, a current flows to a squib 12 through a transistor 22, so that powder 25 is burnt to expand a bag body 27. In the case of the accumulated integrated value is lower than the value judged to be the emergency state of the vehicle and the value to operate the safety sensor 17, the on-off of the safety sensor 17 is detected through wiring 14, and in the case of the safety sensor 17 being off, the failure of the safety sensor 17 is judged.