An optical transmission device, comprising: a symbol demapping unit configured to convert a reception signal to a signal of an I-axis component and a signal of a Q-axis component on a phase plane, and a signal being a concatenation of the signal of the I-axis component and the signal of the Q-axis component; a likelihood generation circuit (15, 16, 17) configured to generate likelihoods relating to the reception signal based on the signal of the I-axis component and the signal of the Q-axis component converted by the symbol demapping unit and on the signal being the concatenation of the signal of the I-axis component and the signal of the Q-axis component; and an error correction decoding unit configured to execute soft decision decoding on the reception signal based on the likelihoods generated by the likelihood generation circuit (15, 16, 17), wherein the likelihood generation circuit (15, 16, 17) includes: a first circuit in form of a first one-dimensional-modulation lookup table configured to input the signal of the I-axis component as an argument to output a first likelihood; a second circuit in form of a second one-dimensional-modulation lookup table configured to input the signal of the Q-axis component as an argument to output a second likelihood; and a third circuit in form of a two-dimensional-modulation lookup table configured to input, as an argument, the signal being the concatenation of the signal of the I-axis component and the signal of the Q-axis component, to generate a third likelihood, characterized in that the error correction decoding unit is configured to execute the soft decision decoding based on the first likelihood, the second likelihood, and the third likelihood, the first one-dimensional-modulation lookup table and the second one-dimensional-modulation lookup table form, with respect to the number of signal points, a low-order rectangular QAM, the two-dimensional-modulation lookup table forms, with respect to the number of signal points, a low-order non-rectangular QAM, and in that the first to third circuits forming the first one-dimensional-modulation lookup table, the second one-dimensional-modulation lookup table, and the two-dimensional-modulation lookup table are shared to be used as a likelihood generation circuit used for, with respect to the number of signal points, a high-order non-rectangular QAM. a symbol demapping unit configured to convert a reception signal to a signal of an I-axis component and a signal of a Q-axis component on a phase plane, and a signal being a concatenation of the signal of the I-axis component and the signal of the Q-axis component; a likelihood generation circuit (15, 16, 17) configured to generate likelihoods relating to the reception signal based on the signal of the I-axis component and the signal of the Q-axis component converted by the symbol demapping unit and on the signal being the concatenation of the signal of the I-axis component and the signal of the Q-axis component; and an error correction decoding unit configured to execute soft decision decoding on the reception signal based on the likelihoods generated by the likelihood generation circuit (15, 16, 17), wherein the likelihood generation circuit (15, 16, 17) includes: a first circuit in form of a first one-dimensional-modulation lookup table configured to input the signal of the I-axis component as an argument to output a first likelihood; a second circuit in form of a second one-dimensional-modulation lookup table configured to input the signal of the Q-axis component as an argument to output a second likelihood; and a third circuit in form of a two-dimensional-modulation lookup table configured to input, as an argument, the signal being the concatenation of the signal of the I-axis component and the signal of the Q-axis component, to generate a third likelihood, characterized in that a first circuit in form of a first one-dimensional-modulation lookup table configured to input the signal of the I-axis component as an argument to output a first likelihood; a second circuit in form of a second one-dimensional-modulation lookup table configured to input the signal of the Q-axis component as an argument to output a second likelihood; and a third circuit in form of a two-dimensional-modulation lookup table configured to input, as an argument, the signal being the concatenation of the signal of the I-axis component and the signal of the Q-axis component, to generate a third likelihood, characterized in that the error correction decoding unit is configured to execute the soft decision decoding based on the first likelihood, the second likelihood, and the third likelihood, the first one-dimensional-modulation lookup table and the second one-dimensional-modulation lookup table form, with respect to the number of signal points, a low-order rectangular QAM, the two-dimensional-modulation lookup table forms, with respect to the number of signal points, a low-order non-rectangular QAM, and in that the first to third circuits forming the first one-dimensional-modulation lookup table, the second one-dimensional-modulation lookup table, and the two-dimensional-modulation lookup table are shared to be used as a likelihood generation circuit used for, with respect to the number of signal points, a high-order non-rectangular QAM.