The superconducting power line is one of the new systems which will be developed to transmit the electric power for a long distance or in populated city. The first purpose of this method is not the leveling up of the electrical efficiency but the density of the transmission energy. There are yet problems to be solved in this line such as whether DC or AC system, the electrical insulation in low temperature, the position of the refrigeration station, the thermal contraction and the technique of protection. Of the many examples the auther introduces the designs of Gauster et al and Garwin et al, and the experimental result of the model which succeeded in BICC in the United Kingdom.