A light modulator of waveguide type (40) comprises electrooptic substrate (1), an optical waveguide (2) for guiding light wave, a traveling wave type signal electrode (3) for controlling the guided light, and a grounding electrode (4). A buffer layer (6), having a width W greater than the width omega of the traveling wave type signal electrode (3), is formed only under traveling wave type signal electrode (3), and the buffer layer is buried at least in part into the surface portion of the substrate (1).