NOVELTY - The modifier has a heat insulating film (4) positioned surrounding a fuel modification unit (2). A water cooling tube (5) is wound in the helical form at the outer peripheral surface of a cylinder (27) surrounding the heat insulating film. The cooling tube absorbs the heat of the fuel modification unit.    USE - For generating modified gas containing hydrogen gas, for supplying to e.g. high output fuel cell.    ADVANTAGE - The heat insulation property with respect to fuel modification unit is improved, and the size and manufacturing cost of the modifier are reduced.    DETAILED DESCRIPTION - A mixing tube (48) mixes the vapor of the modification fuel and raw water from the helical tube (34) of a mixed fluid evaporator (8) and the vapor from the helical tube (51) of raw water evaporator (10) such that the mixed vapor is supplied to the fuel modification unit. A low temperature carbon monoxide transformation catalyst layer (64) is positioned between the inner cylinder (61) and outer cylinder (62) of a double pipe such that the gap at the inner wall face of the double pipe is filled with heat transfer promotion balls (35) are positioned for promoting heat transfer from burner combustion exhaust gas to the helical tube (34). The catalyst layer (64) is positioned surrounding the water cooling tube, for carrying out carbon monoxide transformation of the modified gas generated by the fuel modification unit, such that a gap is maintained between adjacent water cooling tube sections. The burner combustion exhaust gas from the evaporator (8) is sent through the gap between the cooling tube sections such that the catalyst layer is heated. A high temperature carbon monoxide transformation layer (24) is positioned surrounding the fuel modification unit. A cooling unit is positioned extending along perpendicular direction by the side of the carbon monoxide selective oxidation catalyst layer, for cooling the selective oxidation catalyst layer by supplying raw water from lower end of the cooling unit such that the raw water is then supplied to raw water evaporator. The selective oxidation catalyst layer is heated by supplying burner combustion exhaust gas after heating the modification catalyst layer (18) of the fuel modification unit to the waste gas flow path positioned along the side of the selective oxidation catalyst layer. The burner combustion exhaust gas is supplied through the gap of the mixed fluid evaporator after heating the modification catalyst layer. The heat transfer promotion balls are positioned for promoting heat transfer from burner combustion exhaust gas to the helical tube (34). An INDEPENDENT CLAIM is included for modifier operating method that involves measuring maximum temperature of modification catalyst layer of the fuel modification unit during steady operation. The amount of fuel supplied to the burner is controlled to maintain the temperature measurement value in preset temperature range. The vapor temperature of the mixed fluid discharged from the mixed fluid evaporator, and supplied to the fuel modification unit, is measured. The air supply amount to the burner is controlled to maintain the temperature measurement value in preset temperature range.    DESCRIPTION OF DRAWING(S) - The figure shows a sectional view of the modifier. (Drawing includes non-English language text)    Fuel modification unit (2)    Heat insulating film (4)    Water cooling tube (5)    Mixed fluid evaporator (8)    Raw water evaporator (10)    Modification catalyst layer (18)    High temperature carbon monoxide transformation layer (24)    Cylinder (27)    Helical tubes (34,51)    Heat transfer promotion balls (35)    Mixing tube (48)    Inner cylinder (61)    Outer cylinder (62)    Low temperature carbon monoxide transformation catalyst layer (64)