To provide an optical modulator which outputs monitor light for monitoring the state of the output light, from a flank different from a surface where the output light is output, the optical modulator that is small in substrate size and facilitates manufacturing.

An optical waveguide 30 and a waveguide 50 for reflection are provided on the side of one main surface 20a of a semiconductor multilayer substrate 20. The optical waveguide has an input waveguide portion 30a, a modulation waveguide portion 30b, and an output waveguide portion 30c. The input waveguide portion guides input light, input from the side of one first end surface 20b of the semiconductor multilayer substrate. The modulation waveguide portion modulates the input light received from the input waveguide portion to generate a modulated light. The output waveguide portion splits the modulated light, received from the modulation waveguide portion into two first output light and a second output light, and outputs the thus obtained first output light as a signal light from the side of a second end surface 20c opposed to a first end surface. The waveguide for reflecting guides the second output light received from the output waveguide portion to a flank 20d different from a first end surface and a second end surface of the semiconductor multilayer substrate.