An optical device comprises an optical waveguide constituted of as a basic unit a core layer of a material having a relatively high refractive index and a clad layer of a material having a relatively low refractive index and covering said core layer, and heating means for heating said core layer to change the refractive index thereof, characterized in that when said core layer is constituted of a liquid, said core layer is heated by said heating means to a temperature such that said core layer does not boil.