Provided are an apparatus for optimizing bias voltage of an electro-optic modulator, a method therefor, and an optical transmission system using the method. The present research provides a bias voltage optimizing apparatus of an electro-optic modulator that can stabilize bias voltage supplied to the electro-optic modulator through automatic bias voltage initialization control by considering the point that the form of an optical output signal and the intensity of a clock component are changed according to the bias voltage of the electro-optic modulator, a method therefor and an optical transmission system using the method. The apparatus includes: a clock extracting unit for extracting a clock frequency component from the frequency components in a wideband spectrum of the electro-optic modulator; a control unit for finding a point of the lowest electric intensity by detecting an electric intensity of the extracted frequency component and controlling the bias voltage adaptively by changing the bias voltage; and a bias voltage adjusting unit for controlling the bias voltage of the electro-optic modulator under the control of the bias voltage controlling unit.