PURPOSE: To secure a compact and light-weight structure for the microwave circuit by providing the waveguide featuring the reduced width of the face H or E at part on the line, dividing the face H including the center line of face H by the vertical plane and then inserting the slot circuit to the divided face H to be controlled by the converting means.

CONSTITUTION: Cut-off waveguide part 11" featuring the reduced width of face H is provided at part of waveguides 11 and 11', and face H including the center line of face H is divided into two parts by the vertical plane. Then metal-made slot circuit 12 containing mixer diode 13, high-frequency choke 14 and IF output part 15 installed within the slot is inserted into the above divided cavity. At the same time, inductive substance resonator 17 as well as Gunn diode 16 to be the monostabilized local oscillator are mounted to one end of part 11". Furthermore, disk-type high- frequency choke 19 and short-circuit plate 20 are provided at the side of resonator 17. Thus the smooth conversion is ensured via taper 21 provided to circuit 12 from the waveguide to circuit 12. In such way, signal RF sent from the waveguide is converted into signal IF to be sent out through output part 15.