In an end surface 32b of the second transmission line forming body 32 forming a second waveguide 30, the height of a central region 33 which includes an opening of the second transmission line 30b is a reference plane. A depressed portion 32e that is depressed to a depth greater than the length of a thread portion of a screw 205 from the reference plane is provided in a region outside the central region 33 and includes a screw hole forming position. A screw hole 32d for fixing an external circuit 200 to be connected is provided at the screw hole forming position in the depressed portion 32e. The height of a region, which is excluding the depressed portion 32e and is further away from the central region 33 than the screw hole forming position, is equal to the reference plane.