NOVELTY - The concentration distribution estimation device has a concentration distribution calculation unit that computes the concentration distribution of the impurity substance in the anode-side gas channel of a fuel cell based on the amount of impurity substance estimated by an impurity substance estimation unit and the distribution of existence rate of the impurity substance estimated by an impurity substance distribution estimation unit. The gas channel is provided on the anode side of the fuel cell which has an anode and a cathode provided on opposite sides of an electrolyte membrane.    USE - Concentration distribution estimation device for fuel cell in fuel cell system (claimed). Can be used to estimate the concentrating distribution of impurities in an anode-side gas channel of a fuel cell that generates electric power by being supplied with a fuel gas on the anode and with air on the cathode.    ADVANTAGE - Provides an estimation device that is able to accurately estimate the concentration distribution of impurity substances in the anode-side gas channel. Provides a fuel cell system that, by using the estimation device, is able to both prevent the decline in the fuel cell performance resulting from the accumulation of impurity substances in the anode-side gas channel and restrain the amount of the fuel gas emitted to the outside of the fuel cell system. Ensures that the concentration distribution of the impurity substance in the anode-side gas channel can be accurately estimated on the basis of the results of the estimation since the amount of the impurity substance existing in the anode-side gas channel and the distribution of existence rate are calculated by estimation. Ensures that the flow rate distribution of the fuel gas in the anode-side gas channel can be estimated easily and accurately since the electric current value of the fuel cell is measured. Ensures that the permeation amount per unit area of the impurity substance permeating through the electrolyte membrane from the cathode side to the anode side can be easily and accurately estimated since the impedance of the fuel cell is measured. Provides a fuel cell system that can prevent both the accumulation of the impurity substance caused by insufficient emission to reach such a level as to bring about a decline in the fuel cell performance and the wasteful emission of the fuel gas caused by excessive emission. Prevents the decline in the fuel cell performance caused by insufficient emission, and also prevents the waste of the fuel gas caused by excessive emission.    DETAILED DESCRIPTION - The fuel cell generates electric power by being supplied with a fuel gas on the anode, and being supplied with air on the cathode. The impurity substance amount estimation unit calculates by estimation an amount of the impurity substance existing in the anode-side gas channel. The impurity substance distribution estimation unit calculates by estimation a distribution of existence rate of the impurity substance in the anode-side gas channel.    DESCRIPTION OF DRAWING(S) - The drawing is a flowchart showing the routine for estimated calculation of the nitrogen concentration distribution.    Temperature, gas pressure and impedance measurement step (S2)    Standard permeation amount calculation step (S4)    Permeation index calculation step (S6)    Permeation amount calculation step (S8)    Total permeation amount calculation step (S10)    Accumulation amount updating step (S12)