An optical transmitter includes: a light source configured to generate CW light; a drive signal generator configured to generate a drive signal; an optical modulator configured to modulate the CW light with the drive signal so as to generate a first optical signal; a combiner configured to combine the first optical signal and a second optical signal generated by using another light source; and a detector configured to detect a frequency difference between a frequency of the CW light and a center frequency of the second optical signal. The drive signal generator includes: a mapper configured to generate an electric field information signal based on input data; and a frequency controller configured to modify the electric field information signal based on the frequency difference such that the frequency of the CW light matches the center frequency of the second optical signal to generate the drive signal.