NOVELTY - A water vapor modifier (1) has outer chamber (4) packed with water vapor modification catalyst (14) and inner chamber (5) packed with shift catalyst layer (15,16) and mixed catalyst layer (17) having mixture of water vapor modification catalyst and oxidation catalyst. An heat conductive oxygen containing gas supply line (40) penetrating inside the inner chamber centrally from an outlet (9), has gas ejection port (22).    USE - For auto oxidation and modification of the source gas in the presence of water vapor and oxygen, so as to generate hydrogen-rich reformed gas.    ADVANTAGE - Since the oxygen containing gas supply line penetrates the shift and mixed catalyst in the inner chamber, a large heating surface is obtained with small occupancy area of the supply line. Thus size reduced and light weight modifier is obtained. By arranging the heat transfer layers, heat transfer efficiency between the chambers is improved.    DETAILED DESCRIPTION - The modifier performs water vapor modification reaction of the source gas by the heat of combustion, to generate hydrogen rich reformed gas. The mixture of the source gas and the water vapor is oxidized by oxygen containing gas. A partition wall (14a) separates the chambers. Heat transfer layer (14a) in the outer chamber surrounds the shift catalyst layers in the inner chamber. A heat transfer layer (17a) held in the inner chamber, is arranged between the mixed and shift catalyst layers. The modifier (1) is equipped with outer and inner cylinders (2,3) which forms the outer and inner chambers.    DESCRIPTION OF DRAWING(S) - The figure shows a sectional view of the water vapor modifier.    modifier (1)    outer cylinder (2)    inner cylinder (3)    partition wall(4,5) outer and inner chambers(7,9) outlet (3)    water vapor catalyst layer (14)    heat transfer portions (14a,17a)    low and high temperature shift catalyst layers (15,16)    mixed catalyst layer (17)    oxygen containing gas ejection port (22)    oxygen containing gas supply line (40)