PURPOSE:To increase the degree of coupling of plural auxiliary waveguides with a circular main waveguide even to a high frequency exceeding 100GHz so that the electric power made incident to and reflected by the circuit waveguide can be monitored exactly by providing the plural auxiliary waveguides against the circular main waveguide and providing plural sets of coupling holes in each auxiliary waveguide, with each set of the coupling holes being composed of plural holes. CONSTITUTION:On one auxiliary waveguide 3 side in the figure, the coupling holes 2 are provided at intervals l1 and each two adjacent holes constitute one set. A total of N sets of holes 2 is provided at regular intervals l2, with the total number of the holes 2 being 2N. On the other auxiliary waveguide 3a side, the holes 2a are provided at regular intervals l3 and each adjacent holes constitute one set. A total of N sets of holes 2a are provided at regular intervals of l4. When the auxiliary waveguides 3 and 3a are constituted in such way, the TE10 mode can be monitored from the intervals l1 and l2 satisfying a specific relation on one auxiliary waveguide 3 which uses the propagation mode of the main waveguide 1 as a TE04 mode and the incident power can be monitored at an opened end 5b.
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