FUEL CELL SYSTEM

In a fuel cell system, when fully closing an upstream-side valve, a controller performs controlled fully-closed opening-degree control that adjusts the opening degree of the upstream-side valve to a controlled fully-closing opening degree greater than zero by means of a drive mechanism. Upon determining that, during the controlled fully-closed opening-degree control, there is a leakage of oxidant gas in the upstream-side valve, the controller corrects the controlled fully-closed opening degree to the valve-closing side until reaching a zero-position opening degree at which the amount of leakage of the oxidant gas in the upstream-side valve becomes zero.