PURPOSE: To transmit a high frequency signal having a large amplitude and to reduce a transmission loss by forming the line of a trapezoidal sectional shape with a ground layer oppositely to an obtuse angle side of the trapezoidal shape when the line is formed of a superconductor.

CONSTITUTION: An oxide having a composition of, for example, Bi:Si:Ca:Cu = 3:2:2:5 is used as a target on an MgO board 6, and a superconducting film 7 having a 110K layer is grown by using rf sputtering. After a resist pattern 8 of a superconducting transmission line is formed on the film 7, a substrate 6 is inclined to a central axis, irradiated with Ar ions while being rotated, ion milled, the film 7 of a part having no pattern 8 is dry etched to form a transmission line 9 of a trapezoidal shape, and an insulating layer 10, a ground layer 11 are sequentially formed thereon. Thus, the line in which a concentration of an electric field at a corner of the line 9 is alleviated and a transmission efficiency is enhanced is formed.