A digital signal processor-based optical transmitter employing a method such as predistortion or OFDM controls an output of an optical transmission signal at a constant average power at all times even in the event of variation in a peak-to-average power ratio of the optical transmission signal, while allowing an optical transmission at a stabilized S/N ratio. The optical transmitter includes an average power calculation means (30) to calculate an average power of a digital signal output from a digital signal processor circuit (2) and a variable optical power means (31) to keep a constant average power of the optical transmission signal output from an optical vector modulator (5).