To provide a method of purging an anode of a fuel cell system doing away with an inert gas supply device such as a nitrogen gas cylinder in a simple structure.

The method of purging the anode of the fuel cell system is provided with: a reformer part putting water and hydrocarbon fuel under reforming reaction and producing hydrogen-rich gas containing at least hydrogen, carbon monoxide, and lower hydrocarbon; a CO selective oxidizer oxidizing carbon monoxide in the hydrogen-rich gas with oxygen to remove it; and a fuel cell generating power with supply of the hydrogen-rich gas with the carbon monoxide removed to its anode. By producing gas oxidizing and removing hydrogen and components degrading performance of the fuel cell in the hydrogen-rich gas in the CO selective oxidizer, the gas is made circulated in the anode.