A fuel cell system including a fuel cell module and a control device, the fuel cell module comprising: a fuel cell stack formed by stacking a plurality of fuel cells for generating electricity by electrochemical reactions of a fuel gas and an oxygen-containing gas; a partial oxidation reformer for reforming a mixed gas of a raw fuel chiefly containing hydrocarbon and an oxygen-containing gas thereby to produce the fuel gas, and supplying the fuel gas to the fuel cell stack; an exhaust gas combustor for combusting the fuel gas discharged from the fuel cell stack as a fuel exhaust gas and the oxygen-containing gas discharged from the fuel cell stack as an oxygen-containing exhaust gas thereby to produce a combustion gas; and a heat exchanger for raising the temperature of the oxygen-containing gas by heat exchange with the combustion gas, and supplying the oxygen-containing gas to the fuel cell stack, wherein the control device includes a partial oxidation reformer adjuster, a combustion starter, and a power generator; and based on at least any of the reforming state in the partial oxidation reformer determined based on the temperature (T RE) and air/fuel ratio (O 2 /C) therein, and the temperature (T S) of the fuel cell stack, the partial oxidation reformer adjuster adjusts the temperature (T RE) 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, wherein if the temperature (T RE) of the partial oxidation reformer is within a predetermined range, the air/fuel ratio (O 2 /C) is within a predetermined range, and the temperature (T ST) of the fuel cell stack is a predetermined temperature or more, then the power generator starts power generation of the fuel cell stack, characterized in that if the temperature T RE of the partial oxidation reformer is within the range of 700°C ≤ T RE ≤ 900°C, the air/fuel ratio (O 2 /C) is within the range of 0.45 ≤ O 2 /C ≤ 0.55, and the temperature T ST of the fuel cell stack is 650°C or more, (650°C ≤ T ST), then the power generator starts power generation of the fuel cell stack. a fuel cell stack formed by stacking a plurality of fuel cells for generating electricity by electrochemical reactions of a fuel gas and an oxygen-containing gas; a partial oxidation reformer for reforming a mixed gas of a raw fuel chiefly containing hydrocarbon and an oxygen-containing gas thereby to produce the fuel gas, and supplying the fuel gas to the fuel cell stack; an exhaust gas combustor for combusting the fuel gas discharged from the fuel cell stack as a fuel exhaust gas and the oxygen-containing gas discharged from the fuel cell stack as an oxygen-containing exhaust gas thereby to produce a combustion gas; and a heat exchanger for raising the temperature of the oxygen-containing gas by heat exchange with the combustion gas, and supplying the oxygen-containing gas to the fuel cell stack, wherein the control device includes a partial oxidation reformer adjuster, a combustion starter, and a power generator; and based on at least any of the reforming state in the partial oxidation reformer determined based on the temperature (T RE) and air/fuel ratio (O 2 /C) therein, and the temperature (T S) of the fuel cell stack, the partial oxidation reformer adjuster adjusts the temperature (T RE) 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, wherein if the temperature (T RE) of the partial oxidation reformer is within a predetermined range, the air/fuel ratio (O 2 /C) is within a predetermined range, and the temperature (T ST) of the fuel cell stack is a predetermined temperature or more, then the power generator starts power generation of the fuel cell stack, characterized in that if the temperature T RE of the partial oxidation reformer is within the range of 700°C ≤ T RE ≤ 900°C, the air/fuel ratio (O 2 /C) is within the range of 0.45 ≤ O 2 /C ≤ 0.55, and the temperature T ST of the fuel cell stack is 650°C or more, (650°C ≤ T ST), then the power generator starts power generation of the fuel cell stack.