Provided is a fuel cell system capable of a stable start-up operation and stable combustion in a burner unit during the start-up operation of the fuel cell system. During the start-up operation of the fuel cell system, the amount of combustion air delivered to the burner unit (12) by a blower is adjusted according to the aggregate amount of source gas supplied to the burner unit (12) by a source gas supply unit (6) and the estimated amount of source gas out of the source gas components adsorbed in the catalyst of a fuel processor (3) being separated due to the temperature rise of the catalyst and supplied to the burner unit (12).
