A fuel cell system includes a fuel cell module and a control device. The control device includes a partial oxidation reformer adjuster, a combustion starter, and a power generator. Based on at least any of the reforming state in the partial oxidation reformer, the combustion state in the exhaust gas combustor and the temperature of the fuel cell stack, the partial oxidation reformer adjuster adjusts the temperature of the partial oxidation reformer, the supply amount of raw fuel, and the supply amount of oxygen-containing gas, the combustion starter starts combustion in the exhaust gas combustor, and the power generator starts power generation in the fuel cell stack.