NOVELTY - An oxygen-permeating membrane (10) consists of a main layer (12) having oxygen-ion conductivity and electroconductivity in an oxidizing atmosphere, and containing a complex oxide, and a gas-impermeable layer (14) containing a cerium oxide solid solution with rare-earth element as oxide having oxygen ion conductivity, laminated on the main layer. The membrane carries out partial oxidation of reformed fuel. The gas-impermeable layer is formed such that the entire surface of membrane exposed to reformed fuel is covered.    USE - Oxygen-permeating membrane is used in fuel modifiers used for reforming fuel, such as hydrocarbon fuel, to produce hydrogen to be supplied to fuel cells.    ADVANTAGE - The oxygen-permeating membrane has suppressed decrement in performance characteristics even when used in contact with a reducible fluid for reforming reaction.    DETAILED DESCRIPTION - An oxygen-permeating membrane carries out partial oxidation of reformed fuel in surface side (I) while permeating oxygen from surface side (II) to surface side (I), when reformed fuel is supplied to side (I) and oxygen-containing gas is supplied to side (II). The membrane consists of a main layer having oxygen-ion conductivity and electroconductivity in an oxidizing atmosphere, and containing a complex oxide, and a gas-impermeable layer containing a cerium oxide solid solution with rare-earth element as oxide having oxygen ion conductivity as active element, laminated on the main layer. The arrangement is such that the gas-impermeable layer becomes side (I) of the membrane and the main layer becomes side (II). The gas-impermeable layer is formed such that the entire surface of membrane exposed to reformed fuel is covered.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional schematic view of the oxygen-permeating membrane and a schematic view of a modifier.    Oxygen-permeating membrane (10)    Main layer (12)    Gas-impermeable layer (14)    Reformed-fuel flow path (16)    Modifier (20)