To provide an occupant detecting device capable of performing an occupant detection function satisfactorily by warming a sensor itself or its surroundings, even in a cryogenic environment.

This occupant detecting device having an occupant detecting sensor for detecting an occupant on a seat and capable of solving former problems, is equipped with a heater having a function of warming at least the occupant detecting sensor, and a heater control means for controlling the heater so that the temperature of the occupant detection sensor or its surroundings may be a predetermined temperature value or higher. Namely, though the occupant detection function is usually prohibited to operate in the cryogenic environment being outside guarantee temperatures, the sensor itself or its surroundings can be warmed by operating a seat heater or the like, even if the temperature in a vehicle compartment is low and has not risen up to a guarantee temperature, and the occupant detection function is made satisfactorily operatable.