To provide a method whereby a bias voltage capable of easily realizing a required extinction ratio can be controlled by utilizing a high-extinction ratio modulator even when a generic AD converter/control board is used, and a device for realizing the method. In the present invention, the step quantity (variation quantity) ΔV of a control voltage is no more than 0.1 times the half-wave voltage Vπ [V]. For example, in the case of searching for the minimum point, the light intensity is measured when a bias that is larger by the step voltage ΔV and a bias that is smaller by the step voltage ΔV are applied, the current bias voltage being used as a reference, and the bias voltage is moved toward the smaller of the measured light intensities. The process of setting the moved bias voltage as a reference, comparing the light intensities for the bias points in both neighboring positions, and changing the reference bias voltage is then repeated. A configuration may be adopted whereby ΔV then gradually decreases in accordance with a predetermined algorithm.