NOVELTY - The proton electroconductive oxide film-hydrogen permeable film composite film-type electrolyte has a composite film containing hydrogen permeable film (2) and a proton electroconductive oxide film (1). The film (1) has barium or strontium distributed at A-site, and perovskite-type oxide with rare earth element cerium on B-site. The composite film is supported by a porous support (S).    USE - For electrochemical device (claimed), such as phosphoric acid fuel cell, molten carbonate type fuel cell, solid oxide fuel cell and polymer electrolyte fuel cell, hydrogen sensor, hydrogen pump, water vapor sensor and device for exhaust gas purification.    ADVANTAGE - The proton electroconductive oxide film-hydrogen permeable film composite film-type electrolyte has maintained proton electroconductivity, and can be used in mixed gas atmosphere. Anode over-voltage can be reduced. Continuous bondability of the composite surface in proton electroconductive oxide film-hydrogen permeable film composite electrolyte is improved. The electrochemical device utilizing the composite film, has favorable film thickness.    DETAILED DESCRIPTION - The proton electroconductive oxide film-hydrogen permeable film composite film-type electrolyte has a composite film containing hydrogen permeable film (2) and a proton electroconductive oxide film (1). The film (1) has barium or strontium distributed at A-site, and perovskite-type oxide (ABO3) with rare earth element cerium on B-site. The film (1) separates only hydrogen out of the gas which passes through proton and containing carbon dioxide. Film (2) separates only hydrogen out of gas (3) which passes through proton and containing carbon dioxide. Films (1,2) are joined continuously. Film (2) is formed on surface of gas side of film (1). The composite film is supported by a porous support. The rare earth elements are chosen from ytterbium, yttrium, dysprosium, gadolinium, samarium and neodymium. The film (2) is a film of metal or alloy chosen from palladium, niobium, tantalum, lanthanum, titanium, zirconium, copper, nickel and/or silver. An INDEPENDENT CLAIM is included for electrochemical device.    DESCRIPTION OF DRAWING(S) - The figure shows the structure of composite film formed on porous substrate, with cathode and anode. (Drawing includes non-English language text).    proton electroconductive oxide film (1)    hydrogen permeable film (2)    gas containing carbon dioxide (3)    cathode(S) porous support (4)