A fuel cell system includes a fuel cell stack, an oxidizer supply unit, a reformer, a fuel tank, and a water tank. The reformer generates a hydrogen-containing reformed gas reformed from hydrocarbon-based fuel and supplies it to the fuel cell stack. The fuel tank supplies the hydrocarbon-based fuel to the reformer. The water tank supplies water to the reformer. The reformer includes a reforming unit configured to have a reforming reaction generated therein, a combustion unit configured to supply heat energy to the reforming unit, and a carbon monoxide reduction unit configured to reduce the concentration of carbon monoxide in a reformed gas discharged from the reforming unit. A combustion gas pipe is connected to the combustion unit. A reformed gas pipe is disposed between the reforming unit and the carbon monoxide reduction unit. At least one of the combustion gas pipe and the reformed gas pipe is configured to pass through the inside of the water tank or to raise a temperature of the water tank through contact with the water tank, thereby preventing the freezing of the water tank.