NOVELTY - The modifier has a modification reaction unit (1) which forms hydrogen rich gas by water vapor modification reaction. A temperature detector (6) detects the temperature of the modification reaction unit. When the temperature goes below a predetermined value, water vapor supplied to the reaction unit is stopped and gas supply is started. DETAILED DESCRIPTION - A heating gas of hydrocarbon type and water vapor are supplied to modification reaction unit. A shift reaction unit (2) makes CO gas contained in the produced hydrogen rich gas to reduce by an aqueous shift reaction. An oxidation unit (3) selectively oxidizes the CO gas which remains in the hydrogen rich gas. Then, the stoppage of the operation of the modifier is carried out. When the temperature of the modification reaction unit goes below a predetermined value, water vapor supply to the reaction unit is stopped. A gas supply control unit supplies the heating gas to the modification reaction unit. In an alternative embodiment, when modifier is stopped, distribution of gas from the modification reaction unit to the outside of the reaction unit is interrupted. The gas supply control unit supplies the heating gas to the shift reaction unit. The stoppage of the modifier is executed when the temperature of the modification reaction unit goes below a predetermined value. A temperature measuring device measures the temperature of the modification reaction unit and makes the gas supply control unit to operate. The temperature detector has a timer in which the time of carrying out the stoppage of the modifier is set.    USE -   For producing hydrogen rich gas by steam reforming of heating gas of a hydrocarbon type such as methane, propane, butane. The hydrogen rich gas is used as fuel for electricity generation in fuel batteries. DESCRIPTION OF DRAWING(S) - The figure shows the outline of the modifier. (1) Modification reaction unit; (2) Shift reaction unit; (3) Oxidation unit; (6) Temperature detector.    ADVANTAGE -   When the temperature of the modification reaction unit is higher than the temperature at which the heating gas is subjected to thermolysis and carbonaceous precipitation, the modifier is stopped, therefore carbon does not precipitate in the modifier. When the temperature of the modification reaction unit is less than the temperature for thermolysis of the heating gas, supply of water vapor to the reaction unit is stopped, therefore water vapor does not remain in the shift reactor and hence the shift catalyst does not absorb water. Therefore, decomposition of the shift catalyst is avoided. The pressure in each reaction unit is maintained at normal pressure by the heating gas, therefore the shift catalyst is not oxidized, reducing the catalytic property. The temperature detector enables correct switching time of the supply and stoppage of water vapor. The water vapor is quickly removable from the shift reaction unit.