Abstract: A light frequency shifter repeatedly generates, at predetermined cycles, an optical signal, an optical frequency component of which varies in a stepped manner at fixed intervals, and timing generating circuit generates timing signals corresponding to this variation. Detecting mechanisms synchronize, with the timing signal, a frequency difference between a reference light frequency freely selected from the light frequency component and a feedback light frequency of feedback light fed back so as to conform to this reference light frequency, and detect this. Light frequency-regulated light source extracts an output optical signal circulating within a closed loop circuit controlling the light frequency in accordance with the frequency difference described above, and supplies this to a light output terminal, and feeds back a portion of the output optical signal to the detecting mechanism as the feedback light described above.