A coherent transmitter includes a first beam splitter that splits an input first optical signal to obtain a second optical signal and a third optical signal, a first modulator that modulates the second optical signal to obtain a first modulated signal, a phase shift adjustment unit that adjusts a phase of a first sub-signal in the first modulated signal and adjusts a phase of a fourth sub-signal in the phase-adjusted first sub-signal, a first beam combiner that combines a second sub-signal in the first modulated signal and the phase-adjusted fourth sub-signal to obtain a first combined signal, a first PD that performs optical-to-electrical conversion on the first combined signal to obtain a first electrical signal, and a controller that controls, based on the first electrical signal, a voltage applied to the phase shift adjustment unit.