NOVELTY - Fuel (14), steam (17), and air (16) are mixed and supplied to the modification-catalyst layer (6) of a modifier (4), and a reformed gas is produced. The hydrogen generator (1) is equipped with an auxiliary combustor (3) which burns the anode waste gas (26), and heats the fuel, steam or air, and supply of fuel, steam and air is stopped. The waste gas (30) of the auxiliary combustor is supplied to the modifier, and the residual gas of the hydrogen generator is purged.    USE - For stopping hydrogen generators used in fuel cell systems.    ADVANTAGE - The method solves the problems faced during stopping and restarting of hydrogen generator. Since the carbon adhering to the combustion catalyst is removed, the abnormal temperature rise of combustion catalyst during restarting is prevented.    DETAILED DESCRIPTION - Hydrocarbon group fuel (14), steam (17), and air (16) are mixed and supplied to the modification-catalyst layer (6) of a modifier (4). The fuel is modified with steam, and a portion of the fuel is burned in the combustion-catalyst layer (5) of the modifier, and a reformed gas is produced. The carbon monoxide in the reformed gas is decreased by a carbon-monoxide decreasing device, and supplied to the anode of the fuel cell (2). The hydrogen generator (1) is equipped with an auxiliary combustor (3) which burns the anode waste gas (26) of the fuel cell, and heats the fuel, steam or air, and supply of fuel, steam and air is stopped. The waste gas (30) of the auxiliary combustor is supplied to the modifier, and the residual gas of the hydrogen generator is purged.    An INDEPENDENT CLAIM is also included for hydrogen generator.    DESCRIPTION OF DRAWING(S) - The figure shows the fuel cell system. (Drawing includes non-English language text)    Hydrogen generator (1)    Fuel cell (2)    Auxiliary combustor (3)    Modifier (4)    Combustion-catalyst layer (5)    Modification-catalyst layer (6)    Fuel (14)    Air (16)    Steam (17)    Anode waste gas (26)    Waste gas (30)