To provide an automatic frequency control circuit of satisfactory AFC operation in a communication system, mixing not less than two protocols of identical frequency and transmitting it.

The circuit is provided with a frequency difference detecting means 4 for detecting a frequency difference between a received frequency signal 10 and a local transmission signal 12 from a demodulation signal 13 from a wave detecting means 3 and a frame synchronization detecting means 5, for detecting a synchronous word for frame synchronization included in the signal 10 from a demodulation signal 13 from the detecting means 3, and controls the oscillation frequency of a local oscillator 2, based on the output signal 14 of the means 4 and the output signal 15 of the means 5.