An optical waveguide having excellent resistance to moisture, which includes an under cladding layer 2a formed on a substrate 1, core portions 3 formed in a predetermined pattern on the above-mentioned under cladding layer 2a for propagating an optical signal, and an over cladding layer 2b formed so as to cover the above-mentioned core portions 3. At least either the above-mentioned cladding layers 2a and 2b or the core portions 3 are formed of a resin composition containing a polyfunctional oxetane compound having a dicyclopentadiene ring which is represented by the following general formula (1): ##STR00001## wherein the groups R.sub.1 may be the same as or different from each other and each represent a hydrogen atom or an alkyl group having one to six carbon atoms, and n is 0 or a positive number ranging from 1 to 5.