PURPOSE: To reduce the transmission loss by providing a superconductor to each slot of the leakage coaxial cable and bringing the superconductor into the superconducting state to close each slot electrically.

CONSTITUTION: A superconductor 2 and a control element applying the control whether the conductor 2 is brought into the superconducting state or the normal conducting state are provided respectively to slots 1D provided at a prescribed interval along the lengthwise direction of an outer conductor 1A of the leakage coaxial cable 1. Then the superconductor 2 is brought into the superconducting state as required, Thus, the state is equivalent to choked slots, the transmission loss is reduced considerably and the control whether the slots 1D are to be leaked is implemented easily.