PURPOSE: To obtain an extremely stable variable short-circuit unit by inserting the dielectric short-circuit unit which can vary the attachment position in order to have the short-circuit via the face contact.

CONSTITUTION: Variable dielectric short-circuit unit 8 is inserted with a free variable way for the insertion position from open end 6a of low-impedance wave guide 6. And material featuring an extremely high dielectric constant is selected for the dielectric substance for unit 8. As a result, capacity C of the equivalence circuit can be increased, and the impedance can be reduced greatly at the area where unit 8 is inserted against to characteristic impedance Z0 of the circuit. Thus the short-circuit state can be realized.