NOVELTY - Operation of hydrogen generator involves increasing discharge pressure of compressor and adsorption pressure of pressure swing adsorption apparatus (PSA) when temperature of catalyst or combustion exhaust gas exceeds predetermined setting temperature, reducing heating value of gas supplied to burner by improving hydrogen yield in PSA and reducing temperature of modifier to less than setting temperature.    USE - For fuel batteries.    ADVANTAGE - After hydrogen purification, the remaining gases are not wasted or released to the atmosphere. The temperature of the modifier is appropriately controlled. Amount of hydrogen generated is increased while installation costs and operating costs are reduced.    DETAILED DESCRIPTION - A hydrogen rich reformed gas is formed in a reforming pipe (1a) of a modifier (1) having a catalyst layer to which a hydrocarbon raw material gas such as natural gas, liquefied natural gas, liquefied petroleum gas and methanol is passed along with water vapor. The modifier is equipped with a burner (1b) and utilizes combustion exhaust gas as a heating medium of the raw material. A compressor (2) compresses the reformed gas or the raw material gas. A pressure swing adsorption apparatus (3) (PSA) separates and purifies hydrogen from the pressed reformed gas. The remaining gas is supplied and burnt in the burner of the modifier. When the catalyst layer temperature or combustion exhaust gas temperature of the burner exceeds a predetermined setting temperature (which is related to heat resistance temperature of modifier), discharge pressure of the compressor and adsorption pressure of PSA are increased. Heating value of the remaining gas supplied to the burner is reduced by improving the hydrogen yield in PSA. Temperature of the modifier is further reduced to less than setting temperature. An INDEPENDENT CLAIM is also included for a hydrogen generator which comprises a pressure indicator (16) that measures the pressure at discharge end of compressor, a control valve (17) that controls the pressure provided to the by-pass piping path and thermometer (5) to measure the temperature of catalyst layer or combustor exhaust gas. The by-pass piping path is provided to the suction side from the discharge side of the compressor.    DESCRIPTION OF DRAWING(S) - The figure shows schematic block diagram of the hydrogen generator.    Modifier (1)    Reforming pipe (1a)    Burner (1b)    Compressor (2)    Pressure swing adsorption apparatus (3)    Thermometer (5)    Pressure indicator (16)    Controlling valve (17)