To accurately detect the concentration of prescribed gas in air in an outside space of a fuel cell system and conduct appropriate treatment, based on the concentration of the prescribed gas.

Occupant compartment oxygen concentration Cin and exterior oxygen concentration Cout are corrected (S140, S120), by using temperature correction coefficients KTin, KTout based on occupant compartment temperature Tin and exterior temperature Tout and an atmospheric pressure correction coefficient KP based on the atmospheric pressure Pa; when a car is running, the occupant compartment oxygen concentration Cin is compared with thresholds C1, C2, and if the occupant compartment oxygen concentration is lowered, that is informed or the system is stopped (S140-S160); and when the car is not running, the corrected exterior oxygen concentration Cout is compared with thresholds C3, C4, and if car peripheral oxygen concentration is lowered, this is notified or the system is stopped (S170-S190). As a result, oxygen concentration is appropriately detected, and lowering in oxygen concentration is appropriately coped.