To provide an optical transmitter and an optical transmitting method capable of performing a stable optical fiber transmission by subjecting a semiconductor laser light source to a frequency modulation in order to suppress an SBS (induced Brillouin scattering) and mitigating an amplitude fluctuation to be generated at the time of expanding a light spectral line width.

An output light emitted from a semiconductor laser light source 1 is made incident on an external light intensity modulator 2 to be intensity-modulated with a digital signal. The light spectral line width is expanded by minutely subjecting the semiconductor laser light source 1 to a frequency modulation with the output signal of an oscillator 5 in order to suppress the SBS. Since an amplitude modulation is also performed simultaneously with the frequency modulation, an amplitude fluctuation is generated. Here, the digital signal is moreover modulated by varying the gain of a variable gain amplifier 6 by the output signal the oscillator 5. Then, the amplitude fluctuation generated in the semiconductor laser light source 1 can be canceled by driving the intensity modulator 2 with this modulated signal and also by adjusting the delay amount of a delay circuit 7 so that the amplitude fluctuation in the semiconductor light source 1 and the amplitude modulation by the oscillator 5 in the external light intensity modulator 2 become just opposite phases.