A fuel cell system that includes a heat exchange unit which can control the temperature of the fuel and/or the air supplied to a stack unit. In one example embodiment, a fuel cell system includes a reforming unit for generating hydrogen floating gas, and supplying the hydrogen floating gas through a fuel supplying line. The reforming unit includes a burner. The fuel cell system also includes a stack unit for generating electric energy by an electrochemical reaction between air and the hydrogen floating gas. The fuel cell system also includes a heat exchange unit. The heat exchange unit includes a casing having an airtight interior to which off gas generated in the burner is supplied. A portion of the fuel supplying line is arranged to pass through the interior of the casing.