NOVELTY - The fuel cell system comprises a burner part that provides heat to a reformer and is arranged inside the reformer. Hydrocarbon-based fuel and reaction water are introduced into the reformer to generate hydrogen. The hydrogen from the reformer is supplied to a fuel cell stack to produce electricity. The fuel cell stack is installed in an internal space of a hot-water tank. A first heat exchanger is disposed between the hot-water tank and the fuel cell stack. The fuel cell stack is cooled using a first cooling water circulated through a first cooling water circulation pipe.    USE - Fuel cell system.    ADVANTAGE - The fuel cell system has excellent freezing prevention effect and improved thermal efficiency and electric conversion efficiency.    DETAILED DESCRIPTION - The fuel cell system comprises a burner part that provides heat to a reformer and is arranged inside the reformer. Hydrocarbon-based fuel and reaction water are introduced into the reformer to generate hydrogen. The hydrogen from the reformer is supplied to a fuel cell stack to produce electricity. The fuel cell stack is installed in an internal space of a hot-water tank. A first heat exchanger is disposed between the hot-water tank and the fuel cell stack. The fuel cell stack is cooled using a first cooling water circulated through a first cooling water circulation pipe. An interior of the hot water tank is filled with a second cooling water circulated through a second cooling water circulation pipe. The cooling water is an antifreezing solution. The antifreezing solution comprises ethyl alcohol, ethylene glycol, calcium chloride aqueous solution and/or magnesium chloride aqueous solution. The system further comprises three heat exchangers, a second cooling water supply part, and a first cooling water supply part. A reactor is mounted between the reformer and the fourth heat exchanger. A burner exhaust gas outlet line ejects the burner exhaust gas generated by the operation of the burner part. An end of the burner exhaust gas outlet line is connected to the burner part, and another end is connected to the third heat exchanger. The hot-water tank comprises a cooling water inlet installed in its upper end and a hot water outlet, a cooling water outlet and a constant inlet port that are installed in a lower part of the tank. The second cooling water circulation pipe comprises a second cooling water inner circulation pipe and a second cooling water outside return pipe. An end of the second cooling water circulation pipe communicates with the cooling water inlet of the hot-water tank, and another end is connected to the cooling water outlet. A first valve controller controls a first cooling water supply duct including the first cooling water supply part. A second valve controller controls a second cooling water supply duct including the second cooling water supply part. A first cooling water pump pumps the first cooling water. A cooling fan is mounted on the second cooling water circulation pipe for lowering the temperature of the second cooling water passing through the hot-water tank. A second cooling water pump is mounted on the second cooling water circulation pipe, and pumps the second cooling water circulating in an inner portion of the second cooling water circulation pipe. An INDEPENDENT CLAIM is included for a method for driving a fuel cell system.