To reduce a physical size, a price, and power consumption by simplifying a configuration of a Mach-Zehnder type light modulation device.

When a voltage is applied to an electrode 19b of a light modulator 11 by an electric signal and a DC voltage is applied to an electrode 19a from a DC bias source 15, the light inputted to the light modulator 11 from a light source 10 is intensity-modulated by a composite voltage from both electrodes and an interaction, and an optical signal is generated from the light modulator 11. This intensity-modulated light is branched into a optical signal for an external output and an optical signal for monitoring at a branching circuit 12, and the optical signal for monitoring is converted into an electric signal by a PD 13, and based on this electric signal, a DC bias control circuit 14 sets an optimal DC bias value to drift fluctuations of the characteristic of the light modulator 11. In the DC bias source 15, this DC bias value is fed back to the electrode 19a of the light modulator 11.