NOVELTY - A curable composition comprises modified polyorganosiloxane compound (A) having alkenyl group as essential component and triazinane trione derivative, phenolic hydroxyl group or carboxyl group, compound (B) having greater than or equal to 2 thiol group and radical photopolymerization initiator (C).    USE - Curable composition for insulating film used in thin film transistors TFT as passivation film of semiconductor layer or gate insulating film (claimed), transparent material, optical medium, optical lens, optical film, optical sheet, adhesive agent for optical components, optical adhesive agent for optical waveguide bonding, adhesive agent for optical waveguide edge-strip fixing, adhesive agent for digital versatile disk (DVD) bonding, dicing tape, electronic material, printed circuit board, wire coating, high voltage insulating material, interlayer insulation film, the interlayer insulation film for TFT, transparent planarizing film for TFT, packing for insulation, insulation coating material, high heat resistant adhesive agent, high heat dissipation adhesive agent, optical adhesive agent, adhesive agent of LED element, adhesive agent of substrates, adhesive agent of heat sink, coating material, UV powder coating material, ink, coloring ink, UV inkjet ink, hard coat, release paper coating, optical disk coating, optical fiber coating, molding material, fiber reinforced plastic (FRP), sealing material, potting material, sealing material for LED, optical semiconductor sealing material, liquid crystal sealing compound, sealing compound for display devices, sealing material for electric materials, sealing material for solar cells, high heat-resistant sealing material, resist material, liquid resist material, colored resist, dry film resist material, soldering resist material, binder resin for color filters, transparent planarization material for color filters, binder resin for black matrices, photo-spacer material for liquid crystal cells, transparent sealing material for organic LED (OLED) elements, photo-fabrication, solar cell material, fuel cell material, display material, recording material, high-damping material, waterproof packaging, damp-proofing material, heat shrink rubber tube, O-ring, photoconductive drum for copying machines, battery solid electrolyte, gas separation membrane, additive to resins other than a concrete protective material, lining, soil pouring agent, cold storage heat material, sealing material for sterilization apparatuses, contact lens, or oxygen-permeating film.    ADVANTAGE - The composition has excellent optical transparency.    DETAILED DESCRIPTION - A curable composition comprises modified polyorganosiloxane compound (A) having alkenyl group as essential component and triazinane trione derivative of formula (X1)-(X3), phenolic hydroxyl group or carboxyl group, compound (B) having greater than or equal to 2 thiol group and radical photopolymerization initiator (C). INDEPENDENT CLAIMS are included for:    (1) insulating film comprising curable composition; and    (2) thin film transistor comprising insulating film as passivation film of a semiconductor layer or gate insulating film.