PURPOSE: To obtain a stable multi-value modulation output by modulating respectively plural optical outputs distributed by a distribution means and synthesizing modulated optical output.

CONSTITUTION: A laser diode 2 is kept always in the lighting state by a laser diode drive circuit 1 and its optical output is made incident in a waveguide type optical distributer formed on an LiNbO3 board. The optical power is equally distributed by the distributer 7. Waveguide type optical modulators 5, 6 formed on the LiNbO3 board 9 are driven by a voltage from driver circuits 3, 4. Then the optical signal modulated by the waveguide type optical modulators 5, 6 is synthesized by the waveguide type optical synthesizer 8 and sent to an optical fiber 10. Thus, a stable multi-value modulation output is obtained.