A fuel cell system comprising: a fuel cell that generates power by fuel and oxidizer gas; a reforming unit that generates the fuel from a reforming raw material and reforming water and supplies the fuel to the fuel cell; a combustion unit that introduces combustible gas containing unused fuel from the fuel cell thereinto, burns the combustible gas using oxidizer gas, and leads out the combustion exhaust gas; a hot water storage tank that stores stored hot water; a heat exchanger that performs heat exchange between the combustion exhaust gas and the stored hot water, condenses water vapor contained in the combustion exhaust gas, and produces condensed water; a stored hot water circulation line formed to circulate the stored hot water between the hot water storage tank and the heat exchanger; a delivery device that is provided in the stored hot water circulation line and delivers the stored hot water to be circulated; a heat exchanger stored hot water outlet temperature sensor that is provided between a stored hot water lead-out port of the heat exchanger and a stored hot water introduction port (21 c) of the hot water storage tank in the stored hot water circulation line and detects the temperature of the stored hot water led out from the heat exchanger; and a control device that controls the delivery quantity of the delivery device, wherein the control device includes: a water self-sustaining non-establishment determination unit configured to determine whether a water self-sustaining state where a reforming water quantity required for the power generation in the fuel cell is covered by only the condensed water produced by the heat exchanger is established or not, a target temperature decrease setting unit configured to decrease a target temperature of a stored hot water outlet temperature of the heat exchanger in a case where it is determined, by the water self-sustaining non-establishment determination unit, that the water self-sustaining state is not established, and a delivery quantity control unit configured to control the delivery quantity of the delivery device such that the temperature of the stored hot water led out from the heat exchanger becomes the target temperature which is set by the target temperature decrease setting unit. a fuel cell that generates power by fuel and oxidizer gas; a reforming unit that generates the fuel from a reforming raw material and reforming water and supplies the fuel to the fuel cell; a combustion unit that introduces combustible gas containing unused fuel from the fuel cell thereinto, burns the combustible gas using oxidizer gas, and leads out the combustion exhaust gas; a hot water storage tank that stores stored hot water; a heat exchanger that performs heat exchange between the combustion exhaust gas and the stored hot water, condenses water vapor contained in the combustion exhaust gas, and produces condensed water; a stored hot water circulation line formed to circulate the stored hot water between the hot water storage tank and the heat exchanger; a delivery device that is provided in the stored hot water circulation line and delivers the stored hot water to be circulated; a heat exchanger stored hot water outlet temperature sensor that is provided between a stored hot water lead-out port of the heat exchanger and a stored hot water introduction port (21 c) of the hot water storage tank in the stored hot water circulation line and detects the temperature of the stored hot water led out from the heat exchanger; and a control device that controls the delivery quantity of the delivery device, wherein the control device includes: a water self-sustaining non-establishment determination unit configured to determine whether a water self-sustaining state where a reforming water quantity required for the power generation in the fuel cell is covered by only the condensed water produced by the heat exchanger is established or not, a target temperature decrease setting unit configured to decrease a target temperature of a stored hot water outlet temperature of the heat exchanger in a case where it is determined, by the water self-sustaining non-establishment determination unit, that the water self-sustaining state is not established, and a delivery quantity control unit configured to control the delivery quantity of the delivery device such that the temperature of the stored hot water led out from the heat exchanger becomes the target temperature which is set by the target temperature decrease setting unit. a water self-sustaining non-establishment determination unit configured to determine whether a water self-sustaining state where a reforming water quantity required for the power generation in the fuel cell is covered by only the condensed water produced by the heat exchanger is established or not, a target temperature decrease setting unit configured to decrease a target temperature of a stored hot water outlet temperature of the heat exchanger in a case where it is determined, by the water self-sustaining non-establishment determination unit, that the water self-sustaining state is not established, and a delivery quantity control unit configured to control the delivery quantity of the delivery device such that the temperature of the stored hot water led out from the heat exchanger becomes the target temperature which is set by the target temperature decrease setting unit.