In the bias stabilization control using average optical power of signal light, there is a case where a control algorithm (maximum control or minimum control) changes depending on the amplitude of clock voltage, or it becomes out of control. In an optical modulation device for performing CSRZ optical modulation on outgoing light from an optical source, control light inputted from a control light input path is inputted into an LN-MZ optical modulator having traveling-wave type modulating electrodes from a direction opposite to signal light, and the average optical power is detected by a photodetector. A bias-voltage Vb is so generated that this optical power may be minimized, and is applied to a bias input terminal, whereby automatic bias stabilization control is performed.