A re-generator of an optical clock signal has at least a mode-locked semiconductor laser outputting a re-generated optical clock signal from at least one end face of the mode-locked semiconductor laser, with inputted light signals having different polarized wave states respectively at both end faces of the mode-locked semiconductor laser; wherein the re-generator has a light splitter to split the light signal into two kinds of elements with respective polarized waves different by a right angle, and to emit one of the elements split from the light signal, to one of both end faces of the mode-locked semiconductor laser; and a rotator to input the other elements split from the light signal, to rotate the other elements by a right angle, and to lead the rotated element to the other end face of the mode-locked semiconductor laser.