provided is an optical transmission device for transmitting and receiving a multilevel - modulated optical signal , including : a plurality of transmission frame processors for generating transmission frame signals accommodating a plurality of client signals that are each subjected to error correction processing and scrambling / descrambling processing ; and a digital modulator / demodulator for mapping the transmission frame signals that are input to and output from the plurality of transmission frame processors to a multilevel signal and performing digital modulation / demodulation , in which the plurality of transmission frame processors each have a function of shifting a phase of a pattern between a plurality of transmission frames to be mapped to a multilevel signal and be digitally modulated / demodulated .