NOVELTY - A hydrogen generating apparatus has a reforming reaction portion, a modification reaction portion, an air mixer (1), and a selective oxidation reaction portion. The air mixer has an inner tube (12) for supplying selective oxidation air, an outer tube (14) for supplying conversion gas and arranged outer side of inner tube, opening portion for supplying air on circumference of inner tube downstream from supply portion of conversion gas, and annular flow path (15) downstream from opening portion.    USE - Hydrogen generating apparatus is used for fuel cell.    ADVANTAGE - The hydrogen generating apparatus efficiently enables to produce hydrogen. The fuel cell having the hydrogen generating apparatus has high electric power generation efficiency. The carbon monoxide is efficiently removed.    DETAILED DESCRIPTION - A hydrogen generating apparatus has a reforming reaction portion, a modification reaction portion, an air mixer, and a selective oxidation reaction portion. The reforming reaction portion produces hydrogen-containing reformed gas from hydrocarbon and water as raw material. The modification reaction portion reduces carbon monoxide contained in reformed gas by transformation reaction, and produces a conversion gas. The air mixer carries out mixing of conversion gas and selective oxidation air, and produces mixed gas. The selective oxidation reaction portion removes carbon monoxide remaining in mixed gas by selective oxidation reaction. The air mixer has an inner tube for supplying selective oxidation air, an outer tube for supplying conversion gas and arranged outer side of inner tube, opening portion for supplying air on circumference of inner tube downstream from supply portion of conversion gas, and annular flow path downstream from opening portion.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of air mixer. (Drawing includes non-English language text)    Air mixer (1)    Inner tube (12)    Cylindrical portion (13)    Outer tube (14)    Annular flow path (15)