Abstract: The present disclosure stabilizes power generation of a fuel cell stack in an environment in which an oxygen concentration in a fuel cell system readily decreases. In a case where it is detected that the oxygen concentration is lower than a predetermined concentration or where a phenomenon caused by the oxygen concentration being lower than the predetermined concentration is detected, processing to increase a flow rate of air supplied to the fuel cell stack or processing to ventilate the interior of the fuel cell stack by controlling an operation of a ventilation unit is performed.