Abstract: An optical PLL circuit with high precision that has its simplified configuration. An optical-intensity modulator creates difference frequency information (N.times..DELTA.f) by modulating a received optical signal formed of signal optical pulses of a repetitive frequency (N.times.f0) with a reference signal of a frequency (f0+.DELTA.f) and implementing an AND operation of them. The signal is converted into an electric signal by a photo diode. A band-pass filter extracts only the low frequency component (N.times..DELTA.f). A frequency divider produces a frequency component .DELTA.f by dividing the extracted component by N. A multiplier receives the reference signal and the output signal f of a voltage-controlled oscillator and then creates the difference frequency component ((f0+.DELTA.f)-f). A phase comparator compares the low frequency component .DELTA.f with the difference frequency component ((f0+.DELTA.f)-f) and controls the voltage-controlled oscillator to set the phase difference between them to zero.