NOVELTY - The apparatus (1) has a modifier (7) which generates a hydrogen-containing-gas by reforming reaction of source gas and water. A combustor (15) burns the source gas, and heats the modifier. A controller (18) controls the amount of raw material gas supplied by a raw material supplier (11) such that a temperature detected by a temperature detector (10) and controls the amount of water supplied by a water supplier (13) and/or the amount of air supplied by a combustion air supplier (16) based on measured raw material gas flow rate measured by a measurement device (12).    USE - Hydrogen generating apparatus of fuel cell system (claimed).    ADVANTAGE - Since the combustion air is supplied to the combustor, the ratio can become target value, so that the modifier can be stably heated to predetermined temperature with the combustion heat. Since oxidizing agent gas required for the oxidation of carbon monoxide is supplied to the selective-oxidation device, the carbon monoxide concentration in hydrogen-containing-gas can be reduced. The excessive source gas is thermally decomposed by lack of the situation, the water vapor, and carbon component precipitates. The carbon component can prevent the situation where adhere to a modification catalyst and the durability of the hydrogen generating apparatus 1 falls. The reformation water, increases, to reduce the fall of the efficiency of the hydrogen generating apparatus. Thus, the hydrogen generating apparatus can prevent the performance of the hydrogen generating apparatus by the composition change of source gas, and durable fall.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a fuel cell system; and    (2) an operating method of hydrogen generating apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a (Drawing includes non-English language text)    Hydrogen generating apparatus (1)    Modifier (7)    Temperature detector (10)    Raw material supplier (11)    Measurement device (12)    Water supplier (13)    Combustor (15)    Combustion air supplier (16)    Controller (18)