PURPOSE: To suppress the effect of the fluctuation of polarized wave in a single mode fiber by providing a phase modulation element modulating the phase angle at two values of 0 and π to a transmission line of a signal light or a local oscillation light and applying the polarized wave diversity reception in time division.

CONSTITUTION: A phase modulation element 15 is inserted to either a signal optical fiber F1 or a local oscillation optical fiber F2. Then the phase modulation element 15 locates the polarized wave state of the local oscillation light onto a circle passing through the North pole and the South pole of a Poincare sphere, rotates the polarized wave state around an axis perpendicular to the circle to modulate the phase angle by two values of 0 and π. Thus, the sum of the quantity of interference of the polarized wave state PL of the local oscillation light and the polarized wave state PS of the signal light and the quantity of interference of the polarized wave state PS' rotating the polarized wave states PL, PS by π and converting the signal is equal to the quantity of interference when the polarized wave states PL, PS are coincident with each other and the effect of the polarized wave fluctuation in using a single mode fiber for the coherent optical communication is suppressed.