A control device for controlling an electro-optic modulator which receives an optical input signal, modulates the optical input signal with an electrical input signal and a bias voltage signal, and outputs the modulated optical signal is provided. The control device includes: a light source for providing the optical input signal used for the optical modulation to the electro-optic modulator; an optical input measuring unit for measuring the intensity of the optical input signal outputted by the light source; an optical output measuring unit for measuring the intensity of the optical output signal outputted by the electro-optic modulator; and a controller for adjusting the bias voltage signal and the gain of the electrical input signal depending on the intensity of the optical input signal and the optical output signal measured respectively by the optical input measuring unit and the optical output measuring unit. The control device can control the electro-optic modulator by initializing bias voltage and gain of an electrical signal so that the electro-optic modulator can operate in optimized conditions, thus preventing distortion of the optical output signal and obtaining the maximum extinction ratio. In addition, the control device can adjust the bias voltage adaptively during the operation of the electro-optic modulator.