In a PMD compensator of the present invention, a signal light input to an input terminal is supplied to a polarizer via a polarization controller; a part of polarized light transmitted through the polarizer is branched by an optical coupler as a monitor light; the monitor light is converted into a electrical signal by a photodiode; a clock corresponding to a RZ clock frequency of the signal light is extracted from the electrical signal using a band-pass filter, to thereby monitor the intensity thereof; and the polarization controller is feedback controlled by a PC control section so that the monitored intensity of the clock becomes maximum. As a result, a miniaturized and low-cost PMD compensator which can reliably compensate for first-order polarization mode dispersion of a RZ-pulsed signal light is provided.