The invention offers a superconducting-cable core that comprises a superconducting conductor and an insulating layer covering the outer circumference of the superconducting conductor. The superconducting-cable core is classified longitudinally into a cable portion and a jointing-structure-forming portion that is located at each end of the cable portion and that serves as the portion in which a complementary insulating structure is formed when jointed with another conducting member. Of the superconducting-cable core, the jointing-structure-forming portion is at least within the range from the end of the superconducting-cable core to the end of the complementary insulating structure. The jointing-structure-forming portion has an insulating layer whose insulation performance is higher than that of the insulating layer of the cable portion, so that even when the cable itself cannot be designed with a sufficient margin in its insulation, a highly reliable joint can be formed.