PURPOSE: To block a DC current from flowing to a transmission pipe and to prevent damages of high frequency power supplies etc., by forming a choke circuit between choke flanges at the end of the transmission pipe and insulating electrically both choke flanges.

CONSTITUTION: The choke flanges 16a, 16b are provided to joint ends of the transmission pipes 11a, 11b on which high frequency power is transmitted. The flanges 16a, 16b are connected together. The choke circuit 17 is formed between the flanges 16a and 16b. An insulator 20 is inserted between the flanges 16a and 16b to insulate them. Thus, in case when, the titled line is used for the transmission of the high frequency power of plasma heating for a nuclear fusion reactor, the DC current produced in extinction of the plasma can be blocked by the insulator 20. The circuit 17 reduces the electromagnetic wave and high frequency losses leaked externally.