PURPOSE: To eliminate the spurious disturbance, the mutual interference disturbance of local oscillation, etc., caused in a first mixer stage by inserting a trap corresponding to a second local oscillation frequency and changing this trap in accordance with a channel.

CONSTITUTION: A second local oscillation signal is applied to one side of a resonance line 19 from a second local oscillator 10, and a first local oscillation signal is charged to the middle point between mixing diodes 20 and 21 from a first local oscillater 9, and the difference signal between them is taken out from the center of the resonance line and is led to a low-pass circuit 13 by a capacitor 22. A trap 23 corresponding to the second local oscillation frequency is inserted to prevent the second local oscillation signal from going to the first local oscillator. With respect to the trap 23, a parallel trap circuit consists of a resonance line 25 and capacities 26 and 27 and the trap frequency is changed as shown in a figure. Since the second local oscillation frequency is 3GHz, it is trapped by this circuit. Thus, the second local oscillation is suppressed to eliminate the spurious disturbance, the mutual interference disturbance of local oscillation, etc., caused in a first mixer stage.