PURPOSE: To minimize leakage of a microwave other than echoes from an output port, by arranging a plurality of slots or slits almost parallel to a current direction of the microwave on an input port side end face and/or an output port side end face in a hollow resonator.

CONSTITUTION: A slot 23 on an input port side end face 21 is arranged almost parallel to a current direction of a microwave supplied from an input port 4 of a hollow resonator 3 and a slot on an output port side end face 22, almost parallel to a current direction of a microwave generated in a hollow. This enables the mechanically directing of a magnetic field generated perpendicular to an electric field generated in the same direction as a microwave current flowing through an inner wall surface of the hollow resonator 3. This allows efficient introduction of the microwave into the hollow resonator 3 while minimizing leakage of the microwave other than echoes from an output port 5.