NOVELTY - A modifier (300) has a desulfurization unit, a storage unit, a vaporization unit (130), a modification unit (140), a gaseous-phase portion supply unit to supply a portion of gaseous phase stored by storage unit to the modifier, a temperature detection unit to detect the temperature of a modification catalyst, and a control unit to supply portion of gaseous phase to inside of modifier by gaseous-phase portion supply unit, and discharges steam to the modifier exterior, before temperature of the modification catalyst is below the dew point of the steam.    USE - Modifier is used for fuel cell system (claimed), and desulfurizing apparatus. Can also be used in fuel-gas manufacturing apparatus.    ADVANTAGE - The modifier is operated stably. The condensation and decrease of activity of the modification catalyst are suppressed.    DETAILED DESCRIPTION - A modifier has a desulfurization unit to desulfurize a liquid fuel containing hydrocarbon as a main component and to generate a desulfurized fuel, a storage unit which separates the desulfurized fuel into a portion of gaseous phase and a portion of liquid phase, and stores the gaseous and liquid phases, a vaporization unit to carry out supply mixing of steam to portion of liquid phase, and to generate a source gas, a modification unit having a modification catalyst which generates a fuel gas comprising hydrogen as a main component, and which modifies the source gas, a gaseous-phase portion supply unit to supply portion of gaseous phase stored by the storage unit to the modifier, a temperature detection unit to detect the temperature of the modification catalyst, and a control unit to supply portion of gaseous phase to inside of modifier by gaseous-phase portion supply unit, and discharges steam to the modifier exterior, before temperature of the modification catalyst detected by the temperature reduction unit is below the dew point of the steam. When the production of source gas by the vaporization unit is stopped and modification catalyst has dew point more than the dew point of the steam measured by the temperature reduction unit, the steam is supplied to inside of the modifier by the vaporization unit, and the modification catalyst is cooled. INDEPENDENT CLAIMS are included for the following:    (1) fuel cell system; and    (2) operating method of modifier.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of the fuel cell system.    Liquid fuel supply unit (110)    Vaporization unit (130)    Modification unit (140)    Fuel cell (170)    Modifier (300)