An optical transmission apparatus including: - an optical transmitter (100, 600) having a plurality of modulation rules, and being configured to switch between the plurality of modulation rules to generate optical signals, multiplex the optical signals with orthogonal polarizations, and transmit the optical signals, wherein each of the plurality of modulation rules subjects the optical signals to a π/4 shift at each clock cycle, wherein the plurality of modulation rules are switched for each of the orthogonal polarizations, - an optical receiver for receiving said optical signals including: a coherent detector for causing interference between the received optical signals and local oscillation light, and converting the optical signals into electrical signals; a polarization splitter and adaptive equalizer (302, 804) based on a constant modulus algorithm for subjecting the electrical signals, which are input from the coherent detector, to polarization splitting; and a differential detector for performing differential detection on the electrical signals which are input from the polarization splitter and adaptive equalizer (302, 804), wherein the plurality of modulation rules are switched with a switching speed at least ten time greater than a response speed of the polarization splitter and adaptive equalizer (302, 804) based on a constant modulus algorithm. - an optical transmitter (100, 600) having a plurality of modulation rules, and being configured to switch between the plurality of modulation rules to generate optical signals, multiplex the optical signals with orthogonal polarizations, and transmit the optical signals, - an optical receiver for receiving said optical signals including: a coherent detector for causing interference between the received optical signals and local oscillation light, and converting the optical signals into electrical signals; a polarization splitter and adaptive equalizer (302, 804) based on a constant modulus algorithm for subjecting the electrical signals, which are input from the coherent detector, to polarization splitting; and a differential detector for performing differential detection on the electrical signals which are input from the polarization splitter and adaptive equalizer (302, 804), wherein the plurality of modulation rules are switched with a switching speed at least ten time greater than a response speed of the polarization splitter and adaptive equalizer (302, 804) based on a constant modulus algorithm. a coherent detector for causing interference between the received optical signals and local oscillation light, and converting the optical signals into electrical signals; a polarization splitter and adaptive equalizer (302, 804) based on a constant modulus algorithm for subjecting the electrical signals, which are input from the coherent detector, to polarization splitting; and a differential detector for performing differential detection on the electrical signals which are input from the polarization splitter and adaptive equalizer (302, 804), wherein the plurality of modulation rules are switched with a switching speed at least ten time greater than a response speed of the polarization splitter and adaptive equalizer (302, 804) based on a constant modulus algorithm.