NOVELTY - The operation stopping method involves stopping the supply of fuel gas upon detecting an instruction to stop the operation of a fuel cell, then supplying oxide gas to the fuel cell to generate power, and stopping power generation of the fuel cell. After power generation is stopped, a gas replacement apparatus (15) is operated at a predetermined timing to supply replacement gas to the anode side of the fuel cell to replace the fuel gas on the anode side with the replacement gas.    USE - Operation stopping method for fuel cell system.    ADVANTAGE - Enables provision of an operation stopping method in which power is generated by supplying the oxide gas to the fuel cells with the supply of the fuel gas being stopped, so that the hydrogen concentration in the fuel gas in the fuel cells on the anode side drops, thus, hydrogen does not pass through the solid polymer electrolyte membrane toward the cathode side from the anode side, preventing a reaction of hydrogen with oxygen on the cathode side. Suppresses deterioration of the fuel cells since gas replacement is performed at a predetermined timing after power generation is stopped. Enables provision of a fuel cell system with a simplified and compact configuration that effectively suppresses the deterioration of the electrolyte membrane/electrode assembly.    DETAILED DESCRIPTION - The predetermined timing is determined based on an amount of deterioration of the electrolyte membrane/electrode assembly (28) when the gas replacing process is executed at or after a timing at which the power generation of the fuel cell is stopped, and an amount of deterioration of the electrolyte membrane/electrode assembly in a soaked state at or after the timing at which the power generation of the fuel cell is stopped.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic configurational diagram of a fuel cell system.    Fuel cell system (10)    Fuel cell stack (12)    Oxide gas supply apparatus (14)    Gas replacement apparatus (15)    Electrolyte membrane/electrode assembly (28)