NOVELTY - A curable resin composition comprises polysiloxane (A) chosen from group consisting of polyorganosiloxane (A1) having two or more alkenyl groups in a molecule, polyorganosiloxy silalkylene (A2) having two or more alkenyl groups in a molecule, polysiloxane (B) chosen from group consisting of polyorganosiloxanes (B1) having two or more hydrosilyl groups in a molecule and polyorganosiloxy silalkylene (B2) having two or more hydrosilyl groups in a molecule, and imidazole derivative (I).    USE - Curable resin composition is used for forming cured material used as sealing agent for semiconductor device e.g. optical semiconductor device (all claimed). Uses include but are not limited to electronic paper, touchscreen, solar cell substrate, optical waveguide, light-guide plate, electrical insulating material, transparent base material, transparent sheet, laminated board, transparent device, etc.    ADVANTAGE - The curable resin composition does not produce cracks and peeling in sealing material. The cured material obtained using the curable resin composition has excellent heat resistance, transparency, thermal shock resistance, reflow resistance and barrier property with respect to corrosive gas e.g. sulfur oxide, and prevents malfunction of the non-lighting of the optical semiconductor device. The optical semiconductor device sealed using the cured material has high quality and excellent durability.    DETAILED DESCRIPTION - A curable resin composition comprises polysiloxane (A) chosen from group consisting of polyorganosiloxane (A1) having two or more alkenyl groups in a molecule, polyorganosiloxy silalkylene (A2) having two or more alkenyl groups in a molecule, polysiloxane (B) chosen from group consisting of polyorganosiloxanes (B1) having two or more hydrosilyl groups in a molecule and polyorganosiloxy silalkylene (B2) having two or more hydrosilyl groups in a molecule and imidazole derivative of formula (I).    R'a-Rd=H, optionally substituted hydrocarbyl, epoxy group-containing group, oxetanyl group-containing group, hydroxy group-containing group, carboxy group-containing group or mercapto group-containing group, in which one or more of R'a-Rd is alkenyl, epoxy group-containing group, oxetanyl group-containing group, hydroxy group-containing group, carboxy group-containing group and mercapto group-containing group;and    Re,Rf=H alkyl.