A radiation-sensitive resin composition for forming optical waveguides, which comprises (A) a novolac type epoxy resin and (B) a photo-acid generator. The composition is used as materials for a core portion 5 of an optical waveguide 1, and the like. In the composition, component (A) is represented by the following general formula (1). ##STR00001## (In the formula, R.sup.1 is a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, or an aralkyl group; and n is an integer from 0 to 10.) The composition is excellent in patterning properties and the like in curing process, and is also excellent in heat resistance, transmission characteristics, and long-term reliability after the optical waveguide has been formed.