NOVELTY - The hydrogen generator (110) has a modification reactor (112), a switching unit for changing flow path of fluid supplied to reactor and flow path of fluid ejected from reactor, and a controller. A catalyst is heated, and raw material and gas are modified to generate hydrogen-containing gas. The switching unit switches fluid ejection and supply such that modification and exothermic reactions are carried out alternately. The controller changes the reactor from exothermic reaction to modification reaction after reducing oxygen persistence.    USE - For fuel-cell systems (claimed) used for electric-power generation in electric vehicles.    ADVANTAGE - The hydrogen generator supplies a hydrogen-containing gas with low oxygen content ratio, and enables the fuel-cell system to supply stable electric power.    DETAILED DESCRIPTION - The hydrogen generator (110) has a modification reactor (112), a switching unit for changing flow path of fluid supplied to the reactor and flow path of fluid ejected from the reactor, and a controller. When a catalyst is provided, and raw material and gas for modification are supplied, the catalyst is heated and modification reaction of the raw material and gas is carried out to generate a hydrogen-containing gas. When fuel and gas for heat generation are supplied, the catalyst is heated by the exothermic reaction of the fuel and gas. The modification reactor switches the modification reaction and the exothermic reaction. The switching unit switches the fluid ejection and supply such that the modification reaction and exothermic reaction are carried out alternately. The controller changes the reactor from the exothermic reaction to the modification reaction after reducing the oxygen persistence in the reactor. An INDEPENDENT CLAIM is included for fuel-cell system (100).    DESCRIPTION OF DRAWING(S) - The figure shows the schematic drawing of the fuel-cell system. (Drawing includes non-English language text)    Fuel-cell system (100)    Hydrogen generator (110)    Modification reactor (112)    Fuel cell (120)    Fuel-feeding pipe (130A)