Method for Controlling Fuel Cell System and Fuel Cell System

A control method for controlling a fuel cell system includes: setting a fuel composition and a fuel flow rate to a combustor as unknown numbers and setting conditions of an air flow rate to the combustor; estimating composition ratios of the components constituting the mixed gas based on simultaneous equations including at least one of a first characteristic equation based on a relationship between an amount of air supplied to and an oxygen concentration in an exhaust gas discharged from the combustor and a second characteristic equation based on a relationship between the amount of air supplied to and a temperature of the exhaust gas discharged from the combustor, and a composition equation representing a sum of the composition ratios of the types of the components of the mixed gas; and adjusting a flow rate of the fuel supplied to the fuel cell based on the estimated composition ratios.