An optical waveguide-forming material is comprised of a photocurable organopolysiloxane composition comprising an alkali-soluble organopolysiloxane and a photoacid generator, wherein the organopolysiloxane is obtained by (co)hydrolytic condensation of a triorganoxysilane having hydrolyzable epoxide and has an average molecular weight of 500–50,000 as determined by GPC using polystyrene standards. The optical waveguide-forming material, when subjected to pattern formation by photolithography, can be resolved with an alkaline aqueous solution, has a high sensitivity and resolution, and offers a cured film having improved light transmittance, heat resistance and humidity resistance.