<p>A transmission line comprising a low-loss oxide superconductor capable of coping with a large current. A superconductor transmission line has an internal conductor and an external conductor formed of an oxide superconductor. The external conductor has four faces the cross sections of which have a shape that each corner of a hollow square is removed and adjacent ones of which form a slit of under lambda/4 (where lambda is the wavelength of a high frequency to be transmitted)</p>
