PURPOSE: To enable a slave machine and a master machine to perform high grade communication by controlling the switching of transmission power proportionally to the electric field intensity of a reception carrier and stably at either or both of the slave machine and the master machine.

CONSTITUTION: The voltage value which is proportional to the level of the reception carrier in the reception signal from an antenna 1 is outputted by a RSSI circuit 17. In this case, because a reception electric field level is high when a master machine and a slave machine are in a comparatively close distance, the output voltage value of the RSSI circuit 17 becomes higher. Based on this output voltage, a transmission output control circuit 10 controls the coupling of a drive amplifier circuit 9 and a power amplifier circuit 11 to a loose coupling. By this control, transmission power is lowered. On the contrary, because the reception electric field level is small when the master machine and the slave machine are in a distant distance, the output voltage value of the RSSI circuit 17 becomes small, the transmission output control circuit 10 controls the coupling of the drive amplifier circuit 9 and the power amplifier circuit 11 to a close coupling. By this control, transmission power is increased.