A set of first signal light and reference light with a phase difference of almost 0 degree, a set of second signal light and reference light with a phase difference of almost 180 degrees, a set of third signal light and reference light with a phase difference of almost 90 degrees, and a set of fourth signal light and reference light with a phase difference of almost 270 degrees are generated. A first differential signal as a difference between a first light-receiving signal obtained by a first light-receiving element and a second light-receiving signal obtained by a second light-receiving element is calculated, and a second differential signal as a difference between a third light-receiving signal obtained by a third light-receiving element and a fourth light-receiving signal obtained by a fourth light-receiving element is calculated. The first differential signal and the second differential signal are supplied to respective FIR filters. An equalization error is formed from output signals from the FIR filters. Tap coefficients for the FIR filters are controlled to minimize the equalization error.