An optical transmitter of the invention phase modulates an output light from a light source according to data using a phase modulator, and then supplies it to an intensity modulator. The intensity modulator performs pulse carving of the output light from the phase modulator in accordance with a clock signal in which a duty cycle is changed to a value different from 50% by a waveform transformation circuit, with an operating range being a slope from a trough to a crest in the photoelectric response characteristics. As a result, it is possible to realize, at the same time, duty cycle and chirp characteristics that enable excellent transmission characteristics to be obtained.