Fuel cell system and method for inferring wet state of fuel cell

A fuel cell system comprising:

• a fuel cell;
• a reactant gas supply section configured to supply the fuel cell with a fuel gas and an oxidation gas that are reactant gases;
• a converter configured to take out a current and a voltage from the fuel cell toward a load and control the current and the voltage;
• an alternating-current superimposing unit configured to superimpose an alternating-current signal on one of the current and the voltage taken out from the fuel cell by the converter;
• a phase difference deriving unit configured to extract an alternating-current component from an output of the fuel cell and derive a phase difference that is a phase lag of an alternating-current voltage relative to an alternating current in the alternating-current component;
• and
• a first inference unit configured to infer that a wet state of the fuel cell is an inappropriate wet state corresponding to an excessively wet state or an excessively dry state, when, immediately after a magnitude of a change in a value of at least one of parameters that are a flow rate of the reactant gas supplied to the fuel cell, a stoichiometric ratio of the reactant gas, and an output current of the fuel cell, during a predetermined first measurement period is determined to have exceeded a predetermined criterion, an absolute value of an amount of change in the phase difference during a predetermined second measurement period has become equal to or larger than a predetermined criterion value for the phase difference.