Disclosed is a highly stable light modulator with which phase modulation by means of an electro-optic effect occurs on an optical substrate of an electro-optic material of LN or the like, and the setting of an operating point by means of a thermo-optic effect occurs on a planar lightwave circuit (PLC) of quartz, silicon, or the like. By means of this structure, heat is not added directly to the electro-optic material optical substrate, so the influence of thermal drift can be suppressed. Moreover, substrate damage and warping due to heat can be reduced. Furthermore, the quartz used in the PLC has low thermal conductivity, being on the order of one-fifth (approximately 1 W/(m K)) that of the LN substrate, so a desired phase difference can be maintained with low power consumption and the operating point can be stabilized easily.