an automatic frequency control system , for an optical coherent communication system , in which the frequency of local oscillation light 5 from a coherent light communication receiver 100 is swept to pull the intermediate frequency to a stable afc point . the afc is caused to operate with an intermediate frequency in the afc operation range , and interrupted . subsequently while sweeping the frequency of local oscillation light 5 in a certain direction in a predetermined range , the output voltage of frequency discriminator 9 is monitored . once the change of the output voltage of frequency discriminator 9 due to the sweep exceeds the predetermined threshold value , the afc resumes for pulling to a stable point .