FUEL CELL SYSTEM AND OPERATING METHOD THEREFOR

There are provided a fuel cell system capable of reducing thermal stress which acts on a fuel cell at the time of shutdown, and an operating method thereof. A fuel cell system includes a fuel cell (1) that generates electric power using fuel gas and oxygen-containing gas and combusts fuel gas remaining unused for generation of electric power; a fuel gas supply line (9) that supplies the fuel gas to the fuel cell (1); and an on-off valve disposed in the fuel gas supply line (9). A shutdown transition mode in which the fuel gas in the fuel gas supply line (9) downstream from the on-off valve is supplied to the fuel cell (1) at a flow rate smaller than that at a time of generation of electric power and is combusted therein after the on-off valve is closed, and a shutdown mode which is started after the shutdown transition mode are provided as an emergency shutdown mode in which the fuel cell (1) undergoes emergency shutdown when the on-off valve of the fuel gas supply line (9) is closed.