The standard permeation amount of an impurity substance, that is, the permeation amount per unit area of the impurity substance under a standard concentration is calculated from the gas pressures in the gas channels, the impedance, and the fuel cell temperature. The permeation index of the impurity substance at each of locations in the anode-side gas channel is calculated on the basis of the previously calculated value of the concentration distribution of the impurity substance. Then, on the basis of the standard permeation amount and the permeation index, the permeation amounts of the impurity substance at the locations in the anode-side gas channel are calculated. On the basis of a total of the permeation amounts, the amount of the impurity substance accumulated in the anode-side gas channel is calculated. Furthermore, the distribution of existence rate of the impurity substance in the anode-side gas channel is calculated on the basis of the gas pressure in the anode-side gas channel and the electric current value.