A fuel cell system comprising: a fuel cell configured to generate electric power using a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas; a fuel gas supply unit configured to supply the fuel gas to the fuel cell; an oxidizing gas supply unit configured to supply the oxidizing gas to the fuel cell; an oxidizing gas supply passage through which the oxidizing gas is supplied from the oxidizing gas supply unit to the fuel cell; an oxidizing gas discharge passage through which the oxidizing gas unconsumed in the fuel cell is discharged from the fuel cell; an oxidizing gas branch passage configured to branch from the oxidizing gas supply passage; an on-off valve disposed on at least one of the oxidizing gas discharge passage and a portion of the oxidizing gas supply passage, the portion being located between the fuel cell and a branched portion where the oxidizing gas branch passage branches from the oxidizing gas supply passage; an oxidizing gas supply amount measuring unit disposed on the oxidizing gas branch passage and configured to measure a supply amount of the oxidizing gas; and a controller, characterized in that the controller is configured to determine that the on-off valve is normal in a case where the supply amount of the oxidizing gas measured by the oxidizing gas supply amount measuring unit is equal to or larger than a preset first threshold when the controller has activated the oxidizing gas supply unit and output a close signal to the on-off valve, and determine that the on-off valve is abnormal in a case where the supply amount of the oxidizing gas measured by the oxidizing gas supply amount measuring unit is smaller than the first threshold when the controller has activated the oxidizing gas supply unit and output the close signal to the on-off valve. a fuel cell configured to generate electric power using a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas; a fuel gas supply unit configured to supply the fuel gas to the fuel cell; an oxidizing gas supply unit configured to supply the oxidizing gas to the fuel cell; an oxidizing gas supply passage through which the oxidizing gas is supplied from the oxidizing gas supply unit to the fuel cell; an oxidizing gas discharge passage through which the oxidizing gas unconsumed in the fuel cell is discharged from the fuel cell; an oxidizing gas branch passage configured to branch from the oxidizing gas supply passage; an on-off valve disposed on at least one of the oxidizing gas discharge passage and a portion of the oxidizing gas supply passage, the portion being located between the fuel cell and a branched portion where the oxidizing gas branch passage branches from the oxidizing gas supply passage; an oxidizing gas supply amount measuring unit disposed on the oxidizing gas branch passage and configured to measure a supply amount of the oxidizing gas; and a controller, characterized in that the controller is configured to determine that the on-off valve is normal in a case where the supply amount of the oxidizing gas measured by the oxidizing gas supply amount measuring unit is equal to or larger than a preset first threshold when the controller has activated the oxidizing gas supply unit and output a close signal to the on-off valve, and determine that the on-off valve is abnormal in a case where the supply amount of the oxidizing gas measured by the oxidizing gas supply amount measuring unit is smaller than the first threshold when the controller has activated the oxidizing gas supply unit and output the close signal to the on-off valve. determine that the on-off valve is normal in a case where the supply amount of the oxidizing gas measured by the oxidizing gas supply amount measuring unit is equal to or larger than a preset first threshold when the controller has activated the oxidizing gas supply unit and output a close signal to the on-off valve, and determine that the on-off valve is abnormal in a case where the supply amount of the oxidizing gas measured by the oxidizing gas supply amount measuring unit is smaller than the first threshold when the controller has activated the oxidizing gas supply unit and output the close signal to the on-off valve.