A fuel cell system includes a reforming unit, a carbon-monoxide decreasing unit, a fuel cell, a burner unit, and a raw gas supply device. At a start-up operation of the fuel cell system, an amount of raw gas supplied from the raw has supply device is adjusted according to an amount of a desorbed raw gas desorbed out of components of the raw gas adsorbed to at least one of the reforming catalyst and a carbon monoxide decreasing catalyst such that a ratio of an amount of the combustion air to an amount of the raw gas in the burner unit falls within a predetermined range.