PURPOSE: To increase the highest frequency limit and to widen the frequency characteristic by using a balun so as to convert the unbalanced mode into the balanced mode.

CONSTITUTION: Rectangular main lines 4a, 5a are formed to the rear side of a dielectric board 11, an input terminal 2 for a local oscillation signal LO connected with a small capacitance 8a is provided to the outer ridge of the main line 4a and an input terminal 1 of a radio frequency signal RF is provided to the outer ridge of the main line 5a. Strip sub lines 4b, 4c and 5b, 5c in opposition to the main lines 4a, 5a respectively are formed vertically to the surface of the board 11. Then the inner ridge of the sub lines 4a, 4b is connected via a throughhole 4d. The lines 5b, 5a are connected similarly as above. Further, a strap line 7 connects a couple of sub lines 4b, 4c. Thus, the signal LO is converted from the unbalanced mode into the balanced mode by the balun 4 to excite diodes 6a∼6d with the same amplitude but opposite phase. Then the radio frequency signal RF is converted in the reverse mode to above by the balun 5, and inputted to the diodes 6a∼6d with the same amplitude but opposite phase and the result is mixed with the local oscillation signal LO. The signal IF is sent from an output terminal 3 via a line 7. In this case, capacitors 8a∼8c are selected to minimize the conversion loss. Thus, the conversion efficiency is improved and the inter modulation ratio is decreased.