NOVELTY - Introduction of gas into fuel cell system involves generating fuel gas by oxidation reaction and reproducing fuel generation component (1) by reductive reaction, producing electric power by reacting fuel gas supplied from the fuel generation component with oxidizing agent gas containing oxygen in a solid-oxide fuel cell portion (2), and circulating the gas between fuel generation component and fuel electrode (2B) of the fuel cell portion. The introduction of gas into the fuel cell system satisfies a specific relation (I).    USE - Introduction of gas into fuel cell system used for generation of electric power.    ADVANTAGE - The method enables the efficient introduction of gas into fuel cell system with high performance, by simple process. The occurrence of oxidation of metal contained in the fuel electrode of solid-oxide fuel cell portion due to the supply of gas is suppressed.    DETAILED DESCRIPTION - Introduction of gas into fuel cell system involves generating fuel gas by oxidation reaction and reproducing fuel generation component by reductive reaction, producing electric power by reacting fuel gas supplied from the fuel generation component with oxidizing agent gas containing oxygen in a solid oxide fuel cell portion, and circulating the gas between fuel generation component and fuel electrode of the fuel cell portion. The partial pressure ratio of oxidizing gas (OG) is determined by oxidizing the reducing gas (RG). The introduction of gas into the fuel cell system satisfies the relation of formula: f more than v1W/(Po(T2)-P(OG)) (I), where f is the flow volume (sccm) of gas introduced into the fuel cell system, v1 is the reaction rate in fuel generation component reduced using the reducing gas (RG) (sccm/g) at temperature (T1(K)), W is mass (g) in state containing non-oxidized fuel generation component, Po(T2) is the ratio of partial pressure of oxidizing gas (OG) at equilibrium of reverse reaction and oxidation reaction using oxidizing gas (OG) and metal contained in the fuel electrode of solid-oxide fuel cell portion at temperature (T2(K)), T1(K) is the temperature of solid-oxide fuel cell portion, T2(K) is the temperature of fuel generation component, and P is the partial pressure ratio of the reducing gas (RG).    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of the structure of fuel cell system. (Drawing includes non-English language text)    Fuel generation component (1)    Solid-oxide fuel cell portion (2)    Fuel electrode (2B)    Heater (3)    Pump (8)