To provide a fuel cell system that is advantageous for maintaining an S/C value in an appropriate region even when a rotational speed of the water pump is abnormal with respect to a target rotational speed region during a power generation operation of a fuel cell. When the rotational speed of the water pump is abnormal with respect to the target rotational speed region during the power generation operation of the fuel cell, the control unit repeats a short time increase and a short time decrease of the S/C value in a reforming reaction, by alternately repeating an increase in a short time (”T increase, within 10 seconds) and a decrease in a short time (”T decrease, within 10 seconds) of the rotational speed of the water pump with respect to an abnormal rotational speed, while continuing the power generation operation of the fuel cell, thereby averaging the S/C value.
