PURPOSE: To generate stable transmission signals inexpensively by stabilizing a transmission frequency on a basis of a highly stable reception frequency in a radio equipment of the half duplex communication system where signals are transmitted and received with a time difference.

CONSTITUTION: The transmission base band signal inputted from a transmission input terminal 1 passes a switch 2 among switches 2 and 5 turned on only for the transmission time of the radio equipment and a switch 14 turned off for this time and is modulated by an FM modulator 3 and passes a directional coupler 4, a switch 5, and a transmitter and receiver 6 and is transmitted from an antenna 7. Meanwhile, the signal received from the antenna 7 passes the transmitter and receiver 6, a reception band pass filter 8, and a directional coupler 9 and is supplied to a receiving mixer 10. During reception, the FM modulator 3 generates a single sine wave, and this signal is supplied to the receiving mixer 10 through directional couplers 4 and 9 and functions as a reception local signal, and the differential frequency between this local signal and the reception signal is demodulted by an FM demodulator 11. The DC output of the FM demodulator 11 is taken out and is allowed to pass a low pass filter 12 and an amplifier 13 and is fed back to the FM modulator 3 to form a reception following-up AFC loop.