In a polymer waveguide comprising a cladding layer with a low refractive index and a core layer with a high refractive index of a substantially rectangular section covered with the cladding layer, the core layer and a side cladding layer constituting the cladding layer in its portion located on both sides of the core layer are formed of a material comprising a branched polysilane compound containing a silicone compound. By virtue of this constitution, the polymer waveguide has a low loss and causes no significant change in optical characteristics upon a change in ambient temperature.