NOVELTY - A hydrogen generating apparatus (100) having a hydrodesulfurization device (20) which removes sulfur compound in source gas by reacting hydrogen gas and sulfur compound, and a hydrogen generator (2) having a modifier (30) which produces a hydrogen-containing gas using source gas, is operated as follows. Before exhaustion of hydrodesulfurization device, when supplying hydrogen-containing gas to hydrogen utilization apparatus (40), the modifier produces hydrogen-containing gas using the source gas which passes through the hydrodesulfurization device.    USE - Operation of hydrogen generating apparatus in fuel cell systems (claimed).    ADVANTAGE - The method improves the durability of hydrodesulfurization device, and enables continuous supply of desulfurized raw material without requiring exchange of hydrodesulfurization device.    DETAILED DESCRIPTION - A hydrogen generating apparatus provided with a hydrodesulfurization device which removes sulfur compound in source gas by reacting hydrogen gas and sulfur compound, and a hydrogen generator having a modifier which produces a hydrogen-containing gas using source gas, is operated as follows. Before exhaustion of hydrodesulfurization device, when supplying hydrogen-containing gas to the hydrogen utilization apparatus, the modifier produces hydrogen-containing gas using the source gas which passes through the hydrodesulfurization device. After exhaustion of hydrodesulfurization device, when supplying hydrogen-containing gas to the hydrogen utilization apparatus, the modifier produces hydrogen-containing gas using source gas which passes through the normal-temperature desulfurizer from which sulfur compound is removed at normal temperature. An INDEPENDENT CLAIM is included for operating method of fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram if the hydrodesulfurization device of hydrogen generating apparatus. (Drawing includes non-English language text)    Hydrogen generator (2)    Hydrodesulfurization device (20)    Modifier (30)    Hydrogen utilization apparatus (40)    Hydrogen generating apparatus (100)