A light modulation device configured to carry out a quadrature amplitude modulation on a continuous-wave optical signal, comprising: an I-component optical modulator configured to modulate one of continuous-wave optical signals being branched from the continuous-wave optical signal; a Q-component optical modulator configured to modulate other of continuous-wave optical signals being branched from the continuous-wave optical signal; an optical phase shifter configured to apply a phase shift to at least one of an output signal of the I-component optical modulator and an output signal of the Q-component optical modulator; a first bias voltage output part configured to output a first bias voltage being set to a null point when no drive signal is applied to the I-component optical modulator; a second bias voltage output part configured to output a second bias voltage being set to the null point when no drive signal is applied to the Q-component optical modulator; a third bias voltage output part configured to output a third bias voltage used to adjust the phase shift caused by the optical phase shifter; an optical power monitor configured to detect a power of a modulated optical signal modulated by each of the I-component optical modulator and the Q-component optical modulator; a synchronous detection circuit configured to synchronously-detect a component of a frequency f d from the power of the modulated optical signal; and a control circuit configured to output a dither signal of a first frequency to the first bias voltage or the second bias voltage when adjusting the first bias voltage or the second bias voltage while outputting two dither signals having a second frequency, which are mutually orthogonal to each other, applied to the first bias voltage and the second bias voltage when adjusting the third bias voltage, thus increasing or decreasing the bias voltages based on a synchronous detection result, characterized in that the first frequency has a value of f d /n and the second frequency a value of f d / m, where n and m are positive integers equal to or larger than 1 and n<m. an I-component optical modulator configured to modulate one of continuous-wave optical signals being branched from the continuous-wave optical signal; a Q-component optical modulator configured to modulate other of continuous-wave optical signals being branched from the continuous-wave optical signal; an optical phase shifter configured to apply a phase shift to at least one of an output signal of the I-component optical modulator and an output signal of the Q-component optical modulator; a first bias voltage output part configured to output a first bias voltage being set to a null point when no drive signal is applied to the I-component optical modulator; a second bias voltage output part configured to output a second bias voltage being set to the null point when no drive signal is applied to the Q-component optical modulator; a third bias voltage output part configured to output a third bias voltage used to adjust the phase shift caused by the optical phase shifter; an optical power monitor configured to detect a power of a modulated optical signal modulated by each of the I-component optical modulator and the Q-component optical modulator; a synchronous detection circuit configured to synchronously-detect a component of a frequency f d from the power of the modulated optical signal; and a control circuit configured to output a dither signal of a first frequency to the first bias voltage or the second bias voltage when adjusting the first bias voltage or the second bias voltage while outputting two dither signals having a second frequency, which are mutually orthogonal to each other, applied to the first bias voltage and the second bias voltage when adjusting the third bias voltage, thus increasing or decreasing the bias voltages based on a synchronous detection result, characterized in that the first frequency has a value of f d /n and the second frequency a value of f d / m, where n and m are positive integers equal to or larger than 1 and n<m.