NOVELTY - A fuel cell system (100) comprises modifier (8) which produces reformed gas using source gas, fuel cell (10) which produces electric power, hydrodesulfurization apparatus (6), recycle path (9), adsorption desulfurizing apparatus (4), branch path (2) which connects parts of source gas path (1) upstream of adsorption desulfurizing apparatus, adsorption desulfurizing apparatus, and hydrodesulfurization apparatus, switching device (3) which changes source gas circulating or not circulating the adsorption desulfurizing apparatus according to branch path, and heater (7).    USE - A fuel cell system.    ADVANTAGE - The fuel cell system shortens hydrodesulfurization period and effectively reduces desulfurizing agent of adsorption desulfurizing apparatus.    DETAILED DESCRIPTION - A fuel cell system comprises modifier which produces reformed gas using source gas, fuel cell which produces electric power using reformed gas from modifier, hydrodesulfurization apparatus from which sulfur compound in source gas is removed by hydrodesulfurization, recycle path for making part of reformed gas into recycle gas and sending to hydrodesulfurization apparatus, adsorption desulfurizing apparatus from which sulfur compound in source gas is removed by adsorption desulfurization, branch path which connects parts of source gas path upstream of adsorption desulfurizing apparatus, adsorption desulfurizing apparatus, and hydrodesulfurization apparatus, switching device which changes source gas circulating or not circulating the adsorption desulfurizing apparatus according to branch path, and heater which heats hydrodesulfurization apparatus and can be switched on and off.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the fuel cell system. (Drawing includes non-English language text).    Source gas path (1)    Branch path (2)    Switching device (3)    Adsorption desulfurizing apparatus (4)    Hydrodesulfurization apparatus (6)    Heater (7)    Modifier (8)    Recycle path (9)    Fuel cell (10)    Fuel cell system (100)