PURPOSE: To reduce the size of a waveguide by coating a superconduction thin film to the inner face of a basic pipe, and providing a means holding the temperature of the thin film below the transposition temperature of the superconduction substance at the outside of the pipe.

CONSTITUTION: The good conductor thin film 3 on the basic pipe 2 made of aluminum or copper is formed by applying sputtering or electron beam vapor- deposition to a superconduction ceramics having a molecular formula such as Y1Ba2Cu3O7-x. Since the superconduction transposition temperature of the ceramics is 100K, the outer part of the waveguide 1 is cooled by a liquid nytrogen to keep the ceramics thin film at a temperature of ≤100K. As a result, the microwave invation depth (d) becomes smaller and the input angle and reflection angle θ2 are close as large as 45°, then the cut-off frequency of the waveguide 1 is decreased. Thus, the size of the waveguide is reduced.