A process for producing a layered structure containing at least one thin film of oxide superconductor (1) such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x having a contaminated surface on a substrate (3). The contaminated surface of the thin film of oxide superconductor is heat-treated in an atmosphere containing oxygen of high purity of higher than 5N and a partial pressure of 25 Torr at a temperature of 350.degree. to 700.degree. C. On the thin film of oxide superconductor (1), another thin film (2) of oxide superconductor or non-superconductor is deposited.The resulting structure of layered thin films is used for fabricating superconducting transistor, Josephson junctions, superconducting circuits or the like.