An optical waveguide forming epoxy resin composition is free from a diluent component irrelevant to a curing reaction, and comprises: (A) a liquid epoxy resin; and (B) a photoacid generator; wherein the liquid epoxy resin (A) comprises a liquid epoxy resin represented by the following general formula (1) in a proportion of 40 to 75 wt % based on the overall amount of a resin component: wherein R1 and R2 are each a hydrogen atom or a methyl group; R3 to R6 are each a hydrogen atom, a methyl group, a chlorine atom or a bromine atom; X is a C2 to C15 alkylene group, an ethyleneoxy group, a di(ethyleneoxy) group, a tri(ethyleneoxy) group, a propyleneoxy group, a propyleneoxypropyl group, a di(propyleneoxy)propyl group or a tri(propyleneoxy)propyl group; and n is a natural number and has an average value of 1.2 to 5.