PURPOSE: To improve the reflection loss in the operating frequency and to improve the image suppression degree by providing a metallic piece for matching adjustment to a position opposed to a couple of exciting probes respectively.

CONSTITUTION: A conductor foil 7 is projected by a prescribed length on a common plane of a base 3 so as to be orthogonal in the center of holes 4, 5 to constitute a couple of exciting probes 8, 9. The metallic pieces 20, 21 for matching adjustment are formed on the base while being opposed respectively to a couple of the exciting probes 8, 9 and have a capacitance. Since one terminal of the metallic pieces 20, 21 is in contact with the metallic plate 2 in terms of DC, the cut-off frequency is decreased and the reflection loss from the suspended line to the waveguide mode is improved.