PURPOSE: To switch transmission and reception by a pair of a plug and a jack, and to save space by switching supply voltage to be supplied to a microphone and a VOX on the same power supply line for the time of the transmission and for the time of the reception requiring exclusive power supply for the VOX.

CONSTITUTION: When a PTT switch 4a or 4b is in an ON-state, an intermediate voltage generation circuit 10a or 10b impresses the power supply of high voltage to a built-in microphone 5a or an external microphone 5b, and besides, on the reception of the output of the intermediate voltage generation circuit 10a or 10b, a transmission reception decision circuit 20 decides the transmission or the reception, and outputs a transmission detection signal St. Besides, when the PTT switch 4a or 4b is in an OFF-state, a system is turned into a receiving state, and low frequency signal output is inputted to a built-in speaker 8a or an external speaker 8b. At that time, the intermediate voltage generation circuit 10a or 10b impresses the power supply of the voltage lower than that at the time of the transmission to the VOX 30, and the VOX 30 is in a standby state to operate. Thus, the transmission and the reception can be switched by a pair of the plug and the jack.