The fuel cell device has a control device for conducting, at the time of startup, a partial oxidation reforming reaction (POX) inside the reformer, then an auto-thermal reforming reaction (ATR) inside the reformer, then a steam reforming reaction (SR) inside the reformer; a water supply device is provided with a pump for intermittently supplying extremely small amounts of water to the reformer using pulsed control; the control device controls the fuel supply device, the reforming air supply device, and the water supply device to respectively supply target supply flow rates of fuel, reforming air, and water based on the outputs of various sensors; and, in the ATR region, the control device suppresses changes in the flow rate of fuel supplied by the fuel supply device during at least a predetermined interval following the supply of water by the pump.