PURPOSE: To prevent a malfunction due to noise caused at the switching operation from transmission to receiving, by short circuiting the input of the amplifier of an ear microphone for a constant time just before switching operation from transmission to receiving.

CONSTITUTION: The output of ear microphone 1 is input to radio equipment 3 through amplifiers 2A and 2B. The output of mike 1 is input to amplifier 2C, BPF4, amplifier 2D, integrating circuit 9, delay circuit 10, and keeping circuit 11 also, and the output is input to PPT relay driving circuit 11 to drive PPT relay 6. Driving of relay 6 is detected just before driving by PPT detector 12, and the output is input to INST relay driving circuit 7 through keeping circuit 13. INST relay 8 is driven by the output of circuit 7, and inputs to amplifiers 2B and 2C of mike 1 are shorted for a constant time from the time just before switching operation from transmission to receiving.