A fuel cell system comprising: a fuel cell configured to generate electric power using a fuel gas and an oxidizing gas; a fuel gas generator configured to generate the fuel gas containing hydrogen from a raw material; a fuel gas supply path through which the fuel gas is supplied from the fuel gas generator to the fuel cell; a combustor configured to heat the fuel gas generator; a combustion fan configured to supply air to the combustor; an off-gas path through which the fuel gas which has not been used in power generation is supplied from the fuel cell to the combustor; a bypass path connecting the fuel gas supply path to the off-gas path to supply the fuel gas output from the fuel gas generator to the combustor so as to bypass the fuel cell; a flow rate controller configured to control a flow rate of the fuel gas which is output from the fuel gas generator and supplied to the fuel cell and a flow rate of the fuel gas which is output from the fuel gas generator and supplied to the bypass path; and a controller configured to perform a flow dividing step for controlling the flow rate controller such that the fuel gas branches to flow to the fuel cell and to the bypass path, when the fuel gas which is supplied from the fuel gas generator only to the bypass path starts to be supplied to the fuel cell filled with the raw material, characterized in that the fuel cell system is configured to cause the fuel gas to branch so as to flow to the fuel cell and to the bypass path with a flow rate ratio with which an air ratio in the combustor in the flow dividing step is 1 or larger in a state where an air supply amount of the combustion fan is constant before and after the flow dividing step. a fuel cell configured to generate electric power using a fuel gas and an oxidizing gas; a fuel gas generator configured to generate the fuel gas containing hydrogen from a raw material; a fuel gas supply path through which the fuel gas is supplied from the fuel gas generator to the fuel cell; a combustor configured to heat the fuel gas generator; a combustion fan configured to supply air to the combustor; an off-gas path through which the fuel gas which has not been used in power generation is supplied from the fuel cell to the combustor; a bypass path connecting the fuel gas supply path to the off-gas path to supply the fuel gas output from the fuel gas generator to the combustor so as to bypass the fuel cell; a flow rate controller configured to control a flow rate of the fuel gas which is output from the fuel gas generator and supplied to the fuel cell and a flow rate of the fuel gas which is output from the fuel gas generator and supplied to the bypass path; and a controller configured to perform a flow dividing step for controlling the flow rate controller such that the fuel gas branches to flow to the fuel cell and to the bypass path, when the fuel gas which is supplied from the fuel gas generator only to the bypass path starts to be supplied to the fuel cell filled with the raw material, characterized in that the fuel cell system is configured to cause the fuel gas to branch so as to flow to the fuel cell and to the bypass path with a flow rate ratio with which an air ratio in the combustor in the flow dividing step is 1 or larger in a state where an air supply amount of the combustion fan is constant before and after the flow dividing step.