A measuring method first calculates the conductivity of an electrolyte membrane based on measured output voltage and output current of a fuel cell. The water content of an oxidant channel as an index of present water content (PWC) of the fuel cell is calculated based on the calculated conductivity and the other measured physical quantities of the fuel cell. Further the method calculates the water content of the oxidant channel after the inside of the fuel cell reaches a steady state as an index of future water content (FWC) using the measured physical quantities. The PWC index and FWC index are compared, and it is finally determined, based on the result of the comparison and the difference between the previous and present values of the conductivity, whether the water content of the fuel cell is short or excessive at the present time.