NOVELTY - The hydrogen generator has a modification reactor (1), a heater for reactor (1), a shift reactor (13), carbon monoxide removal reactor, air supplier provided at downstream side of removal reactor, and control unit (40). The modification reactor has a modification catalyst (2) for producing reformed gas containing hydrogen, by modification reaction of mixed gas of raw material containing hydrocarbon compound, and water vapor.    USE - For fuel cell system (claimed) used for electric-power generation.    ADVANTAGE - The hydrogen generator enables safe substitution of combustible gas during stoppage. The generator is energy-saving, and has a simple structure.    DETAILED DESCRIPTION - The hydrogen generator has a modification reactor (1), a heater for heating the modification reactor with a gaseous fuel, a shift reactor (13), a carbon monoxide removal reactor, an air supplier provided at the downstream side of the carbon monoxide removal reactor, and a control unit (40). The modification reactor has a modification catalyst (2) for producing a reformed gas containing hydrogen, by modification reaction of a mixed gas of a raw material containing a hydrocarbon compound, and water vapor. The shift reactor has a shift catalyst (12) for producing shift gas. The carbon monoxide in the reformed gas is decreased by a shift reactor, and the carbon monoxide remaining in the shift gas is decreased further. The carbon monoxide removal reactor has a carbon monoxide removal catalyst for producing hydrogen-rich gas. The air supplier supplies air by making the circulation direction of the gas into reference. The control unit supplies air for a preset time from the air supplier, after stopping the supply of raw material and water vapor to the modification reactor. The gas which contains water vapor in order of carbon monoxide removal reactor, shift reactor, and modification reactor, is supplied by the air supply from the air supplier. The shift catalyst is a platinum group shift catalyst.    INDEPENDENT CLAIMS are included for:    (1) a fuel cell;    (2) a method of stopping the hydrogen generator;    (3) a program for functioning as a control unit; and    (4) a recording medium.    DESCRIPTION OF DRAWING(S) - The figure shows the outline block diagram of the fuel cell system. (Drawing includes non-English language text).    modification reactor (1)    modification catalyst (2)    shift catalyst (12)    shift reactor (13)    control unit (40)