To provide a clock extraction method which stably and economically obtains a frequency divided clock signal required for a time division isolation system of optical communication.

A high frequency signal of which the oscillation frequency is changed in accordance with a control signal is generated by a high frequency signal generation part 1 and is given to a phase/wavelength conversion part 2, and an optical wave of which the oscillation wavelength is changed in accordance with the difference between the voltage phase of the high frequency signal and the phase of an optical pulse train of signal light is generated, and an electric signal of which the voltage is changed in accordance with the deviation from a reference value of the wavelength of the optical wave is generated by a wavelength/electric signal conversion part 3, and a prescribed frequency component of the electric signal is taken out by a low frequency selection part 4 and is given to the high frequency signal generation part 1 as the control voltage, and a clock signal is outputted as an electric signal.