A Mach-Zehnder optical modulator with an output light monitoring function whereby emitted light from the modulator is monitored for controlling a DC bias voltage applied between a first and a second electrode provided on a first and a second branch optical waveguide. The end face of the waveguide substrate on the output side is obliquely formed and an optical waveguide is provided on the waveguide substrate for taking out reflected light from the output end face as monitor light. A photodetector for detecting the monitor light is attached to the side face of the waveguide substrate. By such arrangement, the length of the waveguide substrate can be made shorter than that in the prior art. As an alternative, such an arrangement may be made that a double refraction crystal for separating monitor light is attached to the end face on the output side of the waveguide substrate and a prism for correcting optical path is attached to the double refraction crystal.