PURPOSE: To almost eliminate reflection to a microwave incident side by forming a tapered mirror part respectively to the upper-stream of a diverter and the inside wall of a waveguide opposite to the upper-stream end so as to reduce the length.

CONSTITUTION: A metallic diverter 21 is disposed along a center axis in the traveling direction of an incident microwave and formed conically while its diameter is gradually increased from the input side. A tapered mirror section 22 is formed to the outside face of an upper-stream being a small diameter part of the diverter 21. A circular waveguide 23 is provided on the outside of the diverter 21 toward the upper-stream at a prescribed interval and a tapered mirror section 24 is formed opposite to the mirror section 22 of the diverter 21 onto the inside wall of the circular waveguide 23. Then an inside cylinder 25 made of a microwave low loss dielectric body is juxtaposed to the diverter 21 to the tip of the inside wall mirror section 24 of the circular waveguide 23 and the microwave absorbing body is provided on the outside of the inside cylinder 25.