A fuel cell system includes: a polymer electrolyte fuel cell; a resistance sensor that detects the internal resistance of the fuel cell; a dew point sensor that detects the dew point of anode off gas from the fuel cell; and a controller that executes an electrolyte membrane hydration control according to the relationship between the internal resistance and the dew point. According to this configuration, it is possible to define, based on the relationship between the internal resistance and the dew point, the conditions under which the decreased power generation performance of the cell may be quickly recovered.