Provided is a substrate-type optical waveguide, having a phase modulation function, (i) in which a reflection of a signal to be inputted via a coplanar line is restrained and (ii) which consumes less power. In a case where the substrate-type optical waveguide is partitioned into a plurality of sections by cross sections orthogonal to a direction in which light propagates through a core, a local capacitance in each of the plurality of sections gradually increases as a distance from an entrance end surface increases.