NOVELTY - Addition-curable silicone composition comprises: (A) linear organopolysiloxane having at least two silicon atom-bonded aliphatic unsaturated groups and at least one silicone atom bonded substituted hydrocarbon group (I) groups in one molecule; (B) organopolysiloxane having at least two silicon atom-bonded aliphatic unsaturated groups and at least one (I) groups in one molecule, and having either or both of siloxane units as branched structure; (C) an organosilicone compound (II); and (D) platinum group metal-based catalyst.    USE - The addition-curable silicone composition is used as an encapsulant for an optical device (claimed) comprising LED, a semiconductor laser, a photodiode, a phototransistor, a solar battery or charge coupled device.    ADVANTAGE - The addition-curable silicone composition: provides a cured product having a transmittance of light with a wavelength of 300-800 nm at 25 degrees C of 80% or more in a 2 mm thick layer by curing, a refractive index (25 degrees C) with a visible light (589 nm) of 1.40 or less by curing (all claimed), high transparency, excellent light extraction efficiency, excellent reliability, good rubber properties and strength properties and no tack after curing; and improves brightness of LED.    DETAILED DESCRIPTION - Addition-curable silicone composition comprises: (A) 100 parts by mass of a linear organopolysiloxane having at least two silicon atom-bonded aliphatic unsaturated groups and at least one silicone atom bonded substituted hydrofluorocarbon group of formula (I) groups in one molecule; (B) 1-100 parts by mass of an organopolysiloxane having at least two silicon atom-bonded aliphatic unsaturated groups and at least one (I) groups in one molecule, and having either or both of siloxane units of formula (SiO4/2) and (RSiO3/2) as a branched structure; (C) an organosilicone compound of formula (II), where the amount is such an amount that the molar ratio of the silyl hydride (SiH) group in the component (C) to the total aliphatic unsaturated group in the components (A) and (B) satisfies 0.2 less than or equal to SiH group/aliphatic unsaturated group less than or equal to 5; and (D) a platinum group metal-based catalyst.    y = 0 or more;    z = 1 or more;    R = monovalent hydrocarbyl (optionally substituted);    R1 = H or optionally substituted monovalent organic group having 1-10C;    x = 0 or more;    w, p, q = satisfy 1 less than or equal to w less than or equal to 2, 0 less than or equal to p less than or equal to 3, 0 less than or equal to q less than or equal to 3, 2 less than or equal to p+q less than or equal to 3, and w+p+q = 4; and    n = 1 or more and two or more of R1s are H atom.    An INDEPENDENT CLAIM is also included for an optical device encapsulated with a cured product of the addition-curable silicone composition.