NOVELTY - The method involves obtaining a previous estimated value. A factor is obtained, based on the previous estimated value and a value representing a current flow rate of a reactive gas supplied to a fuel cell (10). A period of obtaining the estimated value is multiplied by the factor, so as to calculate a value contributing to a variation in amount of liquid water in the fuel cell. A current estimated value is determined, based on the calculated value.    USE - Method of successively obtaining estimated value of amount of liquid water in fuel cell of fuel cell system (claimed) mounted on fuel cell vehicle. Can also be used in fuel cell system mounted on construction such as house and facility.    ADVANTAGE - The process determines the current estimated value of water content of the fuel cell by taking into account that variation in water content of the fuel cell from the previous period changes depending on the previous amount of liquid water inside the fuel cell, so detection of the water condition in the fuel cell with higher accuracy is enabled. More adequate drainage rate is provided, so the estimation accuracy of the amount of liquid water inside the fuel cell is improved. Correction value is set to be smaller than the difference between the previous estimated value and the reference value, so error of the estimated value is eliminated by repeating correction several times.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a method for obtaining estimated value of amount of liquid water discharged from a fuel cell during operation for specified duration;    (2) a estimation apparatus for successively obtaining estimated value of amount of liquid water in fuel cell during operation; and    (3) a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the fuel cell system.    Fuel cell (10)    Power generating elements (11)    Cathode gas supply assembly (30)    Cathode gas discharge assembly (40)    Refrigerant supply assembly (70)