NOVELTY - Composite oxide film (10) comprises a composite oxide (I), where the composite oxide film has a thickness of 50-500 mm.    USE - The composite oxide film is useful as a solid electrolyte of a solid oxide fuel cell.    ADVANTAGE - The composite oxide film: is thinner than the conventional films, and thus exhibits lower electric resistance and excellent conductivity in the thickness direction; provides the solid oxide fuel cell with improved power generation property; and is easily produced with desired composition ratio.    DETAILED DESCRIPTION - Composite oxide film (10) comprises a composite oxide of formula (A1x(B1)6O1.5x+12), where the composite oxide film has a thickness of 50-500 mm.    A1 = trivalent element;    B1 = tetravalent element; and    x = 6-30.    An INDEPENDENT CLAIM is also included for producing the composite oxide film containing the composite oxide (I), comprising: a first process containing the steps of, onto a substrate, supplying a first material containing one of the elements A1 and B1, supplying an oxidant to form a first layer containing an oxide of the one of the elements A1 and B1, supplying a second material containing another of the elements A1 and B1, and supplying an oxidant to form a second layer containing an oxide of the other of the elements A1 and B1; a second process containing repeating the steps of the first process to prepare a stack of many first layers and many second layers; and a third process containing subjecting the substrate and the stack to a heat treatment to produce the composite oxide film containing (I), where the repetition number ratio between the step of supplying the first material and the step of supplying the second material of the first process is selected to control the composition ratio of the element A1 to the element B1 in the composite oxide film.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic longitudinal sectional view of the composite oxide film immediately after the production.    Composite oxide film (10)    Silicon(100) substrate (12)    Silicon dioxide layer (14)