It is an object of the present invention to provide an optical phase modulation evaluating device that can measure and evaluate the precise degree of modulation in phase of an optical phase modulation signal in comparison with the conventionally-known optical phase modulation evaluating device. The optical phase modulation evaluating module includes: a bit delay device located on optical paths of the third and fifth light beams, and adapted to change the length of the optical paths to delay the third and fifth light beams by one bit per second; and an optical phase difference setting means for delaying either or both the ninth and tenth light beams by a designated phase angle which is not equal to zero, the optical phase difference setting means having a light transmissive plate located on an optical path for the ninth light beam, and a light transmissive plate located on an optical path for the tenth light beam.