PURPOSE: To execute an operation control of a squelch circuit which is not influenced by an interference wave, by detecting a sudden drop of a receiving electric field monitor voltage and operating the squelch circuit.

CONSTITUTION: In a time difference deciding circuit 23 to which outputs of the first comparator 21 and the second comparator 22 are inputted, the time (T) extending from a variation of an output of the first comparator 21 to a variation of an output of the second comparator 22 is counted, and also, the time T is compared with the time (T0) and only when the time T is shorter than the time T0, a control for operating a squelch circuit is sent out of an output terminal Tout. In such a way, by catching a sudden change of a receiving electric field monitor voltage, it can be detected that the signal of a desired wave is disconnected.