Provided is a method for load following operation of a fuel cell system in which reliable reforming and the prevention of flow blockage and anode degradation are possible. Functions F = f(P) and P = f(F) of an electrical output P and a fuel flow rate F required to output P are beforehand obtained. Reforming catalyst layer temperatures Tand reformable fuel flow rates Gat Tare predetermined. Gcorresponding to the maximum Tthat is equal to or less than the measured temperature T of the catalyst layer is set as F. When F< F(minimum F), electric power generation is stopped. When F≥ F, 1 is performed if an output demand value P≤ a maximum output P, and 2 is performed if P> P. 1) In the case of f(P) ≤ F, the output is set to Pand the fuel flow rate is set to f(P). In the case of f(P) > F, the output is set to a value that is less than Pand the maximum among P calculated by P = f(FR), and the fuel flow rate is set to F. 2) In the case of f(P) ≤ F, the output is set to Pand the fuel flow rate is set to f(P). In the case of f(P) > F, the output is set to the maximum value of P calculated from P = f(F), and the fuel flow rate is set to F.
