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.