NOVELTY - A hydrogen generator consisting of a modification-reaction portion (1), a transformation-reaction portion (2), and a heater (4) for heating the modification and transformation reaction portions, is operated as follows. A mixed gas of raw-material-and-mineral-fuel gas and steam is supplied to the modification-reaction portion, while heating the modification-reaction portion and transformation-reaction portion to preset temperature, after the temperature of modification-reaction portion reaches more than carbon-deposition temperature.    USE - Operating method of hydrogen generator used in fuel cells of electric-power generating apparatuses used in motor vehicles, for manufacturing hydrogen from hydrocarbon raw materials, such as natural gas and methanol.    ADVANTAGE - The method maintains the activity of catalyst in the modification-reaction portion and transformation-reaction portion, without using inert gas, such as nitrogen gas, as purge gas during starting of hydrogen generator.    DETAILED DESCRIPTION - A hydrogen generator consisting of a modification-reaction portion (1), a transformation-reaction portion (2), and a heater (4) for heating the modification and transformation reaction portions, is operated as follows. A mixed gas of raw-material-and-mineral-fuel gas and steam is supplied to the modification-reaction portion, while heating the modification-reaction portion and transformation-reaction portion to preset temperature, after the temperature of modification-reaction portion reaches more than carbon-deposition temperature. The modification-reaction portion contains a catalyst consisting of nickel or ruthenium, and produces reformed gas containing hydrogen by steam modification reaction from raw-material-and-mineral-fuel gas and steam. The transformation reaction portion reduces carbon monoxide gas contained in the reformed gas by water-based shift reaction. The modification-reaction portion and the transformation-reaction portion are heated to preset temperature from the state in which running of hydrogen generator is stopped. The mixed gas is supplied to the modification-reaction portion in a state in which the reaction portions are maintained at the reaction temperature. In a state which is below dew point (I) in the transformation-reaction portion when the temperature of the transformation-reaction portion distributes the mixed gas in the starting process, the temperature of modification-reaction portion is maintained under the carbon-deposition temperature which is the temperature at which precipitation of carbon in the surface of modification catalyst is started, when circulating raw materials and mineral fuel in the modification-reaction portion. Raw-material-and-mineral-fuel gas is supplied without supplying steam to the modification-reaction portion. An INDEPENDENT CLAIM is included for hydrogen generator.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the operating method of hydrogen generator. (Drawing includes non-English language text)    Modification-reaction portion (1)    Transformation-reaction portion (2)    Heater (4)    Water supply portion (5)    Fuel supply portion (6)