A fuel cell system comprises: a fuel gas supplier: an air supplier; an air flow rate acquirer that obtains a flow rate of the air that is supplied to a fuel cell; a voltage acquirer that obtains an output voltage of the fuel cell; and a controller. At the time of starting the fuel cell system, the controller controls the fuel gas supplier to supply a fuel gas to the fuel cell for a first time period. After elapse of the first time period, the controller controls the air supplier to start supply of the air. The controller calculates an integrated value of the flow rate for a second time period after the supply of the air is started. When the integrated value becomes greater than a predetermined amount and the output voltage is less than a predetermined voltage, the controller determines that the fuel cell system has an abnormality.