NOVELTY - A modifier (12) modifies raw material gas in modified gas containing hydrogen by heat generation. A preheater (14) carries out partial combustion of modified gas or raw material gas to generate hot gas. A selective oxidation reactor (16) oxidizes CO gas in modified gas selectively. The raw material gas and air containing water vapor are premixed and supplied to partial oxidation modifier.    USE - For modification of fuel used in fuel batteries used in motor vehicle.    ADVANTAGE - Enables rapid starting and load change, and reduces CO concentration in modified gas. Ensure mass production hence reducing cost.    DETAILED DESCRIPTION - The partial oxidation modifier (12), partial oxidation preheater (14) and selective oxidation reactor (16) are arranged in a common container (21) in the order. An indirect heat exchanger (13) which cools gas to suitable temperature for selective oxidation of CO gas, is arranged between partial oxidation modifier and preheater. The modifier has a secondary air feed pipe (12b) which supplies secondary air for partial combustion to the combustion modification catalyst section (12a) filled with catalyst. The raw material gas is related by secondary air. The preheater has a combustion catalyst section (14a) filled with catalyst to which partial combustion air is supplied by combustion air feed pipe (14b). The reactor (16) has a selective oxidation catalyst section (16a) filled with catalyst to which air is supplied by selective oxidation air feed pipe (16b), so that CO gas in modified gas is oxidized selectively. An indirect heat exchanger (11) built in the catalyst section (16a) cools selective oxidation catalyst to a temperature suitable for selective oxidation. Alternately, two or more selective oxidation reactors (16) are connected in series.    DESCRIPTION OF DRAWING(S) - The figure shows block diagram of fuel processing apparatus.    Modifier (12)    Combustion modification catalyst section (12a)    Secondary air feed pipe (12b)    Indirect heat exchangers (11,13)    Preheater (14)    Combustion catalyst section (14a)    Combustion air feed pipe (14b)    Selective oxidation reactor (16)    Selective oxidation catalyst section (16a)    Selective oxidation air feed pipe (16b)    Container (21)