PURPOSE: To reduce influences of ambient noise and radio machine noise to detect a beat disturbance level by providing an intermediate frequency amplifier with a variable band pass filter which detects interference, a detecting circuit which converts it to a DC level, a comparing circuit which compares the variation, and a switching circuit which switches the band characteristic of the band pass filter.

CONSTITUTION: A beat frequency appears in an input level detecting terminal (a) of an intermediate frequency amplifier and detector 15 together with the reception level. The DC component indicating the reception level is given from the input level detection output terminal (a) to a low pass emphasis amplifier 22. Its set level (L dB) is decided by a control part 19, and the control part 19 varies a center frequency (f) of a variable band pass filter 24 to search the beat if the output level is lower than the set level. If the output level is higher than a measured set level (B dB), the frequency of the variable band pass filter 24 is fixed to detect the beat for a certain time, and a signal indicating the presence of interference is outputted to the control part 19 from an output terminal (c). The control part 19 receives this signal to vary the frequency of a VCO 20 to switch the channel to a new channel free from cochannel interference.