Disclosed is a method of forming a thin junction film including a first thin oxide flilm presenting a superconductivity and second thin oxide film presenting an insulator properties or possibly semiconductive properties with an improved production efficiency as well as improved film quality and characteristics. Employed are first and second targets each having substantially the same chemical composition excepting oxygen content. The first target is sputtered at a target cathode voltage of minus 100 V by the use of voltage derived from an external D.C. voltage source, thereby forming a first thin oxide film. Subsequently, the target is changed over to the second target while changing the target cathode voltage into self-bias voltage of minus 50 V without any change of the other film formation conditions. Through this change-over, the sputtering of the first target is successively followed by the sputtering of the second target without any interruption of time, to consequently form the second thin oxide film onto the first thin oxide film.