NOVELTY - A curable resin composition comprises epoxy resin (A) which does not contain an aromatic ring, curing agent (B) or curing catalyst (C), and white pigment (D). The content of the white pigment (D) in the curable resin composition with respect to 100 wt.% of curable resin composition is more 90-99.9 wt.% (excluding 90).    USE - Curable resin composition is used for powder molding for forming hardened material for forming reflector in optical semiconductor element-mounting substrate of optical semiconductor device (all claimed) e.g. electronic paper, touchscreen, solar cell, optical waveguide, light-guide plate and holographic memory. Can also be used for adhesive agent, electric-insulation material, laminated board, coating, ink, coating material, sealant, resist, composite material, sheet, film, optical lens, optical component and optical photofabrication.    ADVANTAGE - The curable resin composition economically provides cured material having high light reflectance, excellent heat resistance, light resistance and toughness, with high production efficiency. The curable resin composition enables molding without carrying out tableting process, with short molding cycle. The cured material has suppressed reflectance fall even when the cured material is left-to-stand at high temperature. The optical semiconductor device formed using the resin composition has excellent durability.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) optical semiconductor element-mounting substrate, which has reflector (100) formed of cured material of the curable resin composition; and    (2) optical semiconductor device, which has optical semiconductor element-mounting substrate, and optical-semiconductor element (107) mounted on the substrate.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of optical semiconductor device.    Reflector (100)    Metal wiring (101)    Package substrate (103)    Bonding wire (104)    Optical-semiconductor element (107)