PURPOSE: To connect a waveguide and parts of a microwave circuit with each other at a low loss, by setting an external conductor for connecting a grounding conductor with a grounding surface in a coaxial line for connecting an input- output terminal with an external connecting conductor.

CONSTITUTION: Packaged parts 3 of a microwave circuit, whose input-output terminal 1 is extruded downward from the lower surface of a printed board 4 after passing through a grounding surface 2 is mounted on the printed board 4. The terminal 1 is connected with a probe 13 in a waveguide 5 by the printed board 4 and a coaxial line 9 passing through a grounding conductor 9. The coaxial line 9 is set in a concave part 14 provided in the conductor 7. When such a constitution is used, deterioration of the noise characteristics of this microwave circuit is reduced, since the line 9 has a smaller loss as compared with a strip line 6. Moreover, fluctuation of the matching between the inside of the waveguide 5 and the parts 3 can be suppressed smaller.