NOVELTY - Fuel cell system includes dry-up controller (62) to decide that a solid polymer membrane is in a dry-up state when a temperature difference between a temperature of coolant and outlet temperature of oxidant gas exceeds a first threshold value.    USE - Fuel cell system i.e. stationary or portable type fuel cell system useful for fuel cell vehicle, motorcycles, or robots.    ADVANTAGE - The stack dry-up is decided based on the temperature difference between the temperature of the coolant and the outlet temperature of the oxidant gas, so that it can be decided that the generated water is easily vaporized and the humidified water amount in the fuel cell is decreased to start the stack dry-up. Regardless of the change in the dew point of the oxidant gas supplied to the fuel cell, it can be precisely and immediately decided that the stack dry-up is started. Unlike the related art, the stack dry-up can be decided without additionally installing the dew point detector, so that the manufacture cost can be prevented from being increased.    DETAILED DESCRIPTION - Fuel cell system comprises fuel cell to generate electric power using fuel gas and oxidant gas supplied to the fuel cell, where fuel cell comprises solid polymer membrane, fuel electrode, and oxidant electrode; fuel gas passage through which the fuel gas passes along the fuel electrode in the fuel cell; oxidant gas passage through which the oxidant gas passes along the oxidant electrode in the fuel cell; coolant passage via which a coolant flows into the fuel cell to adjust a temperature of the fuel cell; oxidant gas outlet temperature detector to detect an outlet temperature of the oxidant gas discharged from an outlet of the oxidant gas passage; coolant temperature detector to detect a temperature of the coolant passing through an inlet or an outlet of the coolant passage; and dry-up controller (62) to decide that the solid polymer membrane is in a dry-up state when a temperature difference between the temperature of the coolant and the outlet temperature of the oxidant gas exceeds a first threshold value.    DESCRIPTION OF DRAWING(S) - The drawing is a block diagram of electric control unit.    Temperature difference calculation mechanism (61)    Dry-up controller (62)    Dry-up prevention control unit (63)    System dry-up decision mechanism (66)    Stack dry-up decision mechanism (67)