Superconducting thin film and a method for preparing the same

A high-temperature superconducting thin film composed of compound oxide and deposited on a substrate of MgO or SrTiO.sub.3, wherein said compound oxide is a member selected from the group consisting of:

• Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Ho.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Lu.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,
• Sm.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Nd.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Gd.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,
• Eu.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Er.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Dy.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,
• Tm.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Yb.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, La.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,
• (La, Sr).sub.2 CuO.sub.4-x,
• in which x is a number which satisfies a range of 0<x<1, and wherein an outer surface of said high-temperature superconducting thin film is covered with a protective layer comprising a crystalline film which is composed of SrTiO.sub.3.