In an optical multilevel transmitter (210), a polar representation of an optical multilevel signal (r, φ) is generated by a polar coordinate multilevel signal generation circuit (212), input to an optical amplitude modulator (211) and a polar coordinate type optical phase modulator (201), and output as an optical multilevel modulated signal (213). The polar coordinate type optical phase modulator (201) generates an optical phase rotation proportional to an input voltage, so the modulation distortion of the electric signal is transferred in a linear form to the optical phases of the optical multilevel modulated signal (213). In an optical multilevel receiver (219), a received signal is input to two sets of optical delay detectors (133) and balance receivers (134) and directly demodulated, and a differential phase Δφ for the received signal is calculated by arctangent computation from the output signal. In a phase adaptive equalizer (205), the modulation distortion of the phase is removed by adaptive equalization of the differential phase Δφ. By separately receiving the amplitude components and combining them, the modulation distortion is removed and highly sensitive optical multilevel transmission is achieved.