PURPOSE: To avoid interruption of head of speech by adding a delay circuit to the pre-stage of a talking sound volume changeover switch of a loud talking control circuit.

CONSTITUTION: A transmission delay circuit 3 is arranged between a microphone amplifier 2 and a transmission sound volume changeover switch 4. In this case, a sound signal to a transmission sound volume detector 11 is extracted between the circuit 3 and the amplifier 2. The voice inputted to a microphone 1 is inputted to the circuit 3 via the amplifier 2, a prescribed value of delay is given and the result is inputted to the switch 4. Thus, since the transmission voice signal is inputted after the switch 4 is changed over sufficiently when the mode is changed over from the reception to the transmission, the interruption of head of speech is avoided.