The present invention provides a photosensitive epoxy resin composition which contains: (A) a cresol novolak polyfunctional epoxy resin; (B) a liquid epoxy resin having a fluorene skeleton in its main chain; and (C) a photoacid generator; wherein the components (A) and (B) are present in a mixing weight ratio of (A)/(B)=40/60 to 60/40. Where the inventive photosensitive epoxy resin composition is used as an optical waveguide forming material, particularly as an optical waveguide core layer forming material for formation of a core layer, the optical waveguide core layer can be formed as having excellent reflow resistance and a lower loss through a coating process and a roll-to-roll process without altering the conventional production process.