An optical transmitter includes: a data generating unit configured to generate a plurality of modulating signals; a driver 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 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 convert 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.