An optical modulator and an optical transmission system convert continuous light of a multiple wavelength light source, which generates the continuous light with a fixed and complete phase but different frequencies, to a modulator driving signal so as to generate a light subcarrier with each frequency at the center and modulate the continuous light to the light subcarrier by using the modulator driving signal. In the case where an optical modulation is carried out by an optical IQ-modulator, transmitting data, for example, is converted to two parallel data A(t) and B(t), an I phase signal, in which the data A(t)+B(t) are modulated with a clock signal with a frequency ω, and a Q phase signal, in which the data A(t)−B(t) are modulated with a clock signal with a π/2 phase shifted, are generated, and the I phase signal and the Q phase signal are applied to electrodes of the optical IQ-modulator, respectively.