An optical transmitter, comprising: - a data generating unit configured to generate a plurality of modulating signals; - a driver (8,9) configured to amplify the plurality of modulating signals generated by the data generating unit; - a phase shifter configured to control a phase of at least one signal among the plurality of modulating signals to be input to the driver; - a plurality of optical modulators (1,2) connected in series to each other, and configured to modulate an optical signal on a basis of each of the modulating signals amplified by the driver; - an optical coupler configured to branch the optical signal modulated by the optical modulator arranged at a last stage in the series; - a photodiode configured to detect the optical signal branched by the optical coupler and converts the optical signal into an electric signal; and - a phase control unit configured to control an amount of phase control of the phase shifter to maximize an intensity of the electric signal converted by the photodiode and further comprising: - a temperature monitoring unit configured to detect temperature of an optical fiber transmitting the optical signal; and - a phase control amount storage unit configured to store both the temperature detected by the temperature monitoring unit and the amount of phase control of the phase control unit, both the temperature and the amount of phase control corresponding to a point when the intensity of the electric signal converted by the photodiode indicates maximum value, - wherein, when the temperature detected by the temperature monitoring unit is stored in the phase control amount storage unit, the phase control unit reads the amount of phase control corresponding to the detected temperature from the phase control amount storage unit and controls the phase shifter according to the read amount of phase control. - a data generating unit configured to generate a plurality of modulating signals; - a driver (8,9) configured to amplify the plurality of modulating signals generated by the data generating unit; - a phase shifter configured to control a phase of at least one signal among the plurality of modulating signals to be input to the driver; - a plurality of optical modulators (1,2) connected in series to each other, and configured to modulate an optical signal on a basis of each of the modulating signals amplified by the driver; - an optical coupler configured to branch the optical signal modulated by the optical modulator arranged at a last stage in the series; - a photodiode configured to detect the optical signal branched by the optical coupler and converts the optical signal into an electric signal; and - a phase control unit configured to control an amount of phase control of the phase shifter to maximize an intensity of the electric signal converted by the photodiode and further comprising: - a temperature monitoring unit configured to detect temperature of an optical fiber transmitting the optical signal; and - a phase control amount storage unit configured to store both the temperature detected by the temperature monitoring unit and the amount of phase control of the phase control unit, both the temperature and the amount of phase control corresponding to a point when the intensity of the electric signal converted by the photodiode indicates maximum value, - wherein, when the temperature detected by the temperature monitoring unit is stored in the phase control amount storage unit, the phase control unit reads the amount of phase control corresponding to the detected temperature from the phase control amount storage unit and controls the phase shifter according to the read amount of phase control.