NOVELTY - A fuel cell system (1) comprises a fuel cell stack (2); cell voltage detector (17); total voltage detector (18); cell voltage difference computing unit (21, 22); stack current detector (14); and anode outlet local current detector (13). A current density-computing unit (20) computes a stack current density and an anode outlet local current density. A wet state-judging unit (23) judges a wet state of the fuel cell stack based on the difference between average cell voltage and minimum cell voltage, and based on the difference between stack current density and anode outlet local current density.    USE - Fuel cell system for vehicles.    ADVANTAGE - The wetting state of the fuel cell can be accurately determined without degrading the reliability of the system.    DETAILED DESCRIPTION - A fuel cell system (1) comprises a fuel cell stack (2) including single cells. Each single cell includes an anode to which fuel gas can be supplied, a cathode to which oxidizing gas can be supplied, and a solid polymer electrolyte membrane interposed between the anode and the cathode. The oxidizing gas and the fuel gas are configured for counter-flow with respect to each other. A cell voltage detector (17) detects a cell voltage at a cathode outlet-side of a single cell. A total voltage detector (18) detects a total voltage of the fuel cell stack. A cell voltage difference computing unit (21, 22) determines an average cell voltage from the total cell voltage detector and a minimum cell voltage from the cell voltage detector and computes a difference between the average cell voltage and the minimum cell voltage. A stack current detector (14) detects electric current in the fuel cell stack. An anode outlet local current detector (13) detects a local current at an anode outlet-side of the fuel cell stack. A current density-computing unit (20) computes a stack current density from the stack current detector and an anode outlet local current density from the anode outlet local current detector. A wet state-judging unit (23) judges a wet state of the fuel cell stack based on the difference between the average cell voltage and the minimum cell voltage, and based on the difference between the stack current density and the anode outlet local current density. An INDEPENDENT CLAIM is also included for a method for controlling a fuel cell stack.    DESCRIPTION OF DRAWING(S) - The figure shows a system configuration diagram of a fuel cell system.    Fuel cell system (1)    Fuel cell stack (2)    Anode outlet local current detector (13)    Stack current detector (14)    Cell voltage detector (17)    Total voltage detector (18)    Current density-computing unit (20)    Cell voltage difference computing unit (21, 22)    Wet state-judging unit (23)