A digital optical coherent transmission device for an optical transmission system using one of a multivalued modulation method and a frequency division multiplexing method, comprising: an O/E converter configured to optical-coherent receive a received optical signal and to convert the optical signal into an electric signal; an analog/digital conversion unit configured to convert the electric signal from the O/E converter into a digital signal; a first constellation acquisition unit configured to acquire first constellation information for the output digital signal of the analog/digital conversion unit; characterized by further comprising: an amplitude control unit configured to control amplitude of the electric signal from the O/E converter when it is determined according to the first constellation information, that the amplitude of the output digital signal is not in a preset optimum range; a digital signal processing unit provided at a later stage of the first constellation acquisition unit and configured to demodulate a signal by performing digital signal processing on the output digital signal; and a second constellation acquisition unit configured to acquire second constellation information on a signal demodulated by the digital signal processing unit, wherein it is detected whether or not amplitude of the demodulated signal is in the preset optimum range according to the second constellation information, and when it is not in the preset optimum range, the amplitude control unit is directed to control amplitude of an electric signal from the O/E converter. an O/E converter configured to optical-coherent receive a received optical signal and to convert the optical signal into an electric signal; an analog/digital conversion unit configured to convert the electric signal from the O/E converter into a digital signal; a first constellation acquisition unit configured to acquire first constellation information for the output digital signal of the analog/digital conversion unit; characterized by further comprising: an amplitude control unit configured to control amplitude of the electric signal from the O/E converter when it is determined according to the first constellation information, that the amplitude of the output digital signal is not in a preset optimum range; a digital signal processing unit provided at a later stage of the first constellation acquisition unit and configured to demodulate a signal by performing digital signal processing on the output digital signal; and a second constellation acquisition unit configured to acquire second constellation information on a signal demodulated by the digital signal processing unit, wherein an amplitude control unit configured to control amplitude of the electric signal from the O/E converter when it is determined according to the first constellation information, that the amplitude of the output digital signal is not in a preset optimum range; a digital signal processing unit provided at a later stage of the first constellation acquisition unit and configured to demodulate a signal by performing digital signal processing on the output digital signal; and a second constellation acquisition unit configured to acquire second constellation information on a signal demodulated by the digital signal processing unit, wherein it is detected whether or not amplitude of the demodulated signal is in the preset optimum range according to the second constellation information, and when it is not in the preset optimum range, the amplitude control unit is directed to control amplitude of an electric signal from the O/E converter.