A fuel cell system having a fuel cell, a fuel supply flow path for allowing a fuel gas supplied from a fuel supply source to flow to the fuel cell, an on-off valve for regulating the state of the gas on the upstream side of the fuel supply flow path to supply it to the downstream side, failure detection means for detecting whether the on-off valve has a closure failure, a pressure sensor for detecting the pressure value of the fuel gas on the upstream side of the on-off valve, and determination condition setting means for setting, based on the pressure value detected by the pressure sensor, the conditions of determination of the closure failure of the on-off valve. The failure detection means determines whether the on-off valve has the closure failure, based on the closure failure determination conditions set by the determination condition setting means.