An object is to provide a light modulator capable of highly accurate bias control by maintaining main output characteristics and monitor characteristics in an appropriate relationship and matching a bias point determined using monitor output and an optimal bias point of main output. The light modulator includes an optical waveguide formed in a substrate having a thickness of 20 μm or less, in which the optical waveguide includes a Mach-Zehnder waveguide and an output waveguide for guiding signal light from a multiplexing portion of the Mach-Zehnder waveguide and outputting the signal light outside the substrate and monitoring means that monitors signal light or radiated light. Leaked light-removing means for removing some of the radiated light propagating through the output waveguide from the output waveguide and emitting the radiated light outside the substrate is provided in the light modulator.