A semiconductor optical modulator, comprising: an optical interference unit for branching an input light signal into two optical waveguides (11, 12), combining respective branched light signals propagating through the respective optical waveguides (11, 12) and outputting the combined light signal; a high-frequency voltage supply for supplying a high-frequency voltage (Vpp) to one optical waveguide out of the two optical waveguides (11, 12), wherein the high-frequency voltage supply is operable to supply a low level high-frequency voltage and a high level high-frequency voltage, the difference between the high level high-frequency voltage and the low level high-frequency voltage having a fixed amplitude, irrespective of the wavelength of the input light; a bias voltage supply for supplying a bias voltage (Vb) to the said one optical waveguide supplied with the high-frequency voltage by the high-frequency voltage supply; a bias voltage controller for controlling a value of the bias voltage (Vb) supplied by said bias voltage supply, in accordance with the wavelength of the input light; a phase adjusting voltage supply for supplying a phase adjusting voltage (Va) for adjusting the phase of one of the branched light signals propagating through at least one optical waveguide of the two optical waveguides (11, 12); and a phase adjusting voltage controller for controlling a value of the phase adjusting voltage (Va) supplied by the phase adjusting voltage supply; characterised in that: said bias voltage controller is operable to control the value of the bias voltage (Vb) so that the phase difference between the respective branched light signals changes by an amount of n between an off level and an on level of the combined light signal; and said phase adjusting voltage controller is operable to control the value of the phase adjusting voltage (Va) supplied by the phase adjusting voltage supply so as to adjust the phase of one of the branched light signals propagating through at least one optical waveguide of the two optical waveguides (11, 12) so that the phase difference between the respective branched light signals becomes zero at the low level or the high level of the high-frequency voltage (Vpp) supplied by the high-frequency voltage supply, and the combined light signal of an on level is outputted. an optical interference unit for branching an input light signal into two optical waveguides (11, 12), combining respective branched light signals propagating through the respective optical waveguides (11, 12) and outputting the combined light signal; a high-frequency voltage supply for supplying a high-frequency voltage (Vpp) to one optical waveguide out of the two optical waveguides (11, 12), wherein the high-frequency voltage supply is operable to supply a low level high-frequency voltage and a high level high-frequency voltage, the difference between the high level high-frequency voltage and the low level high-frequency voltage having a fixed amplitude, irrespective of the wavelength of the input light; a bias voltage supply for supplying a bias voltage (Vb) to the said one optical waveguide supplied with the high-frequency voltage by the high-frequency voltage supply; a bias voltage controller for controlling a value of the bias voltage (Vb) supplied by said bias voltage supply, in accordance with the wavelength of the input light; a phase adjusting voltage supply for supplying a phase adjusting voltage (Va) for adjusting the phase of one of the branched light signals propagating through at least one optical waveguide of the two optical waveguides (11, 12); and a phase adjusting voltage controller for controlling a value of the phase adjusting voltage (Va) supplied by the phase adjusting voltage supply; said bias voltage controller is operable to control the value of the bias voltage (Vb) so that the phase difference between the respective branched light signals changes by an amount of n between an off level and an on level of the combined light signal; and said phase adjusting voltage controller is operable to control the value of the phase adjusting voltage (Va) supplied by the phase adjusting voltage supply so as to adjust the phase of one of the branched light signals propagating through at least one optical waveguide of the two optical waveguides (11, 12) so that the phase difference between the respective branched light signals becomes zero at the low level or the high level of the high-frequency voltage (Vpp) supplied by the high-frequency voltage supply, and the combined light signal of an on level is outputted.