PURPOSE: To generate reference timing signals which can highly sensitively detect respective components of two, three and four times harmonic frequency fm from interference light by using general components.

CONSTITUTION: A clock signal is sequentially divided by 2 by first and second dividing means 33, and its output having inverse polarity is divided by 8 by a third dividing means 36 to obtain a reference signal of 3fmr. The inverse polarity output of the first dividing means 33 is divided by 3 by a fourth dividing means 43, and its output is divided by 8 by fifth and sixth dividing means 45, 46 to obtain a reference signal of 2fmr. The inverse polarity output of the fifth dividing means 45 is divided by 2 by a seventh dividing means 51 to obtain a reference signal of 4fmr. The fourth dividing means 43 comprises a JK flipflop, while other dividing means comprise D-type flipflops.