The object of the present invention is to provide an optical transmitter which enables the reduction of the strong dependency of the transmission characteristic on the high-speed polarization scrambler driving signal phase. The light outputted from a light source is converted to a RZ pulse by an electro-absorption modulator. The RZ pulse is modulated by a data modulator, and the polarization thereof is scrambled in a high-speed electro-optic polarization scrambler and outputted from the output terminal thereof. Since, in the present invention, the light is once changed to a RZ pulse, the strong dependency of the optical signal transmission characteristic on the high-speed polarization scrambler driving signal phase can be reduced. Further, if the phase of the RZ pulse in the NRZ modulation signal is made to synchronize with the rising edge or the falling edge of the NRZ signal, the dependency on the high-speed polarization scrambler driving signal phase can be further reduced.