A transmission line is provided which has a low loss and can flow large current. A superconductor transmission line has: an internal conductor; and an external conductor surrounding the internal conductor, made of oxide superconductor and having four planes, which four planes have a cross section of a hollow quadrilateral with each corner portion being removed, and adjacent planes of which define a slit narrower than .lambda./4 (.lambda. being a wavelength of a high frequency wave to be transmitted).