PURPOSE: To attain the control of the reception/transmission electric power in response to the receiving situation of a transmitting destination by changing the transmission output of a radio signal at a transmission means in accordance with a received control signal.

CONSTITUTION: A reception level detecting means 14 detects the level of a received signal and outputs a control signal to a control signal transmission means 18 in response to the detected level The means 18 of a transmitter/ receiver A transmits the control signal to a transmitter/receiver B by radio. At the transmitter/receiver A, the control signal is received from the transmitter/receiver B by a control signal reception means 16 and then sent to a transmission output changing means 20. The means 20 changes the transmission output of its own transmitter/receiver in response to the control signal received from the transmitter/receiver B. Thus the undesired power is never consumed at transmission and therefore a transmitter/receiver driven by a battery can perform the communication for a longer period of time.