NOVELTY - A fuel cell system (100) comprises a desulfurizer (2) from which sulfur component in source gas is removed at normal temperature, desulfurizer (3), a modifier (4) which produces reformed gas which contains hydrogen using source gas, a source gas path (7) for source gas supplying to one of desulfurizers, a recycle path (6) for sending to desulfurizer (3) by making into recycle gas portion of reformed gas produced with modifier, a branch path (8) which branches from the source gas path of upstream of one desulfurizer, and merges in source gas path between desulfurizer and modifier.    USE - Fuel cell system is used for production of hydrogen.    ADVANTAGE - The fuel cell system has excellent ability to activate the hydrodesulfurization agent, and enables efficient production of hydrogen.    DETAILED DESCRIPTION - A fuel oil system comprises a desulfurizer (2) from which sulfur component in source gas is removed at normal temperature, desulfurizer (3) from which the sulfur component is hydrogenated and removed, a modifier which produces the reformed gas which contains hydrogen using source gas with which the sulfur component is removed with either desulfurizer (2) and desulfurizer (3), a source gas path used for source gas supply to one of desulfurizers, a recycle path for sending to desulfurizer (3) by making into recycle gas a portion of reformed gas produced with the modifier, a branch path which branches from the source gas path of the upstream of the one desulfurizer, and merges in the source gas path between the one desulfurizer and the modifier. The fuel cell electric-power-generated using the reformed gas produced with the modifier. The other of desulfurizer (2) and desulfurizer (3) is positioned at the branch path. The source gas is supplied to the modifier via desulfurizer (2) until the hydrodesulfurization agent of the desulfurizer (3) is activated by the reformed gas from the recycle path after installation of the fuel cell system, or maintenance. The source gas is supplied to the modifier via desulfurizer (3) after the hydrodesulfurization agent of desulfurizer (3) is activated.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of fuel cell. (Drawing includes non-English language text)    Desulfurizer (2,3)    Modifier (4)    Fuel cell (5)    Recycle path (6)    Source gas path (7)    Branch path (8)    Fuel cell system (100)