In a waveguide type optical device which has an optical waveguide and modulation electrodes for varying its refractive index, both formed in the top of a substrate and a ferroelectric crystal having a pyroelectric effect, the top of the crystal substrate being parallel to the direction of its spontaneous polarization, conductive films are formed in two surfaces of the crystal substrate which cross the direction of the spontaneous polarization. The conductive films are electrically interconnected to thereby prevent a change in the operating temperature characteristic of the optical device which is caused by the pyroelectric effect of the crystal substrate.