The present invention provides a method for forming an optical waveguide characterized by applying a paste containing a copper compound to a glass substrate containing an alkali metal as a glass component over the whole surface thereof or in a patterned form, and performing heat treatment at a temperature lower than the softening temperature of the glass substrate.The method of the invention can produce an optical waveguide without the need for a high vacuum as in the thin film deposition method and without the use of a molten salt, and is capable of dispersing Cu.sup.+ ions selectively in a glass substrate with excellent controllability.