NOVELTY - Epoxy resins and a photoinitiator are blended to obtain a photocurable resin composition. The composition is applied on a substrate and the solvent is volatilized, to form a resin layer. A flat plate (7) is press-contacted in parallel with the substrate surface in a state in which the resin layer is heated. The resin composition is cooled and solidified, and the flat plate is spaced apart to form a primary molded product. Selective hardening is carried out, followed by removal of unexposed portion, to obtain a fine resin structure.    USE - Manufacture of fine resin structure e.g. optical waveguide, used in manufacture of photoelectric circuit boards (all claimed) used in microelectromechanical systems and system on chip. Can also be used for manufacturing optical lenses, diffraction grating, biochips, and polymer spring structures.    ADVANTAGE - The method enables manufacture of fine resin structures having high flatness and uniform height, even on surface of substrates having step difference like a groove on the surface. The method enables manufacture of low-loss optical waveguides excellent in flatness, adhesiveness, flowability and developability.    DETAILED DESCRIPTION - An epoxy resin which is a liquid at normal temperature is blended in preset ratio with an epoxy resin which is a solid at normal temperature. A photoinitiator is further blended to obtain a photocurable resin composition which is a solid at normal temperature, and melted or softened by heating. The composition is applied on a substrate surface and the solvent is volatilized, to form a solid photocurable resin layer. A flat plate is press-contacted in parallel with the substrate surface in a state in which the resin layer is heated to melting or softening temperature. In the press-contact state, the resin composition is cooled to a temperature at which the composition is solidified. A primary molded product having a flat surface is formed by spacing the flat plate apart. Selective hardening of the region by masking and exposing portion except the region to be hardened from the flat surface side of the primary molded product, is carried out. A fine resin structure is then obtained by carrying out image-development removal of unexposed portion of the selectively hardened primary molded product. An INDEPENDENT CLAIM is included for manufacture of photoelectric circuit board (1), which involves using a silicon substrate (2) having a groove (3) for optical-waveguide (4) formation where a silicon oxide film (5) is formed in the surface, applying the photocurable resin composition to cover the groove, volatilizing solvent to form a resin layer for core (4a), press-contacting the flat plate with the substrate, solidifying the resin composition, forming a resin layer (8) for non-hardened core by spacing the flat plate apart, selectively hardening region of resin layer, forming a core by removing the unexposed portion by image development, and forming a clad (10b) by covering the core with a resin having a refractive index lower than that of the core.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional schematic view explaining the manufacturing method of optical waveguide.    Photoelectric circuit board (1)    Silicon substrate (2)    Groove (3)    Optical waveguide (4)    Core (4a)    Silicon oxide film (5)    Non-hardened resin layer (8)    Clad (10b)