NOVELTY - An addition-curable silicone composition comprises organopolysiloxane (I) (100 parts mass) having viscosity of 30 Pa.second or more at 25 degrees C, cyclosiloxane compound (II) (0.5-100 parts mass), organohydrogenpolysiloxane (III) in which at least two hydrogen atoms are bonded with silicon atom in a molecule, and hydrosilylation catalyst containing platinum type metal. The molar ratio of hydrogen atom bonded with the silicon atom of component (III) with respect to the aliphatic unsaturated group bonded with the silicon atom of component (I) and component (II) is 0.2-10.    USE - Addition-curable silicone composition is used as sealing material for optical elements (claimed) e.g. light-emitting diode, semiconductor laser, photodiode, phototransistor, solar cell and charge coupled device.    ADVANTAGE - The addition-curable silicone composition has excellent heat resistance, cold resistance, electric-insulation property, and heat-resistance color-change property. The optical element using the composition has excellent transparency.    DETAILED DESCRIPTION - An addition-curable silicone composition comprises organopolysiloxane of formula: R1-SiO(-R2)2-(SiO(Me)2)n-Si(-R2)2-R1 (I) (100 parts mass) having viscosity of 30 Pa.second or more at 25 degrees C, cyclosiloxane compound of formula (II) (0.5-100 parts mass), organohydrogenpolysiloxane of formula: R53-SiO(-SiO(Me)(-H))p(SiO(-Me)2)q-SiR53 (III) in which at least two hydrogen atoms are bonded with silicon atom in a molecule, and hydrosilylation catalyst containing platinum type metal. The molar ratio of hydrogen atom bonded with the silicon atom of component (III) with respect to the aliphatic unsaturated group bonded with the silicon atom of component (I) and component (II) is 0.2-10.    Me=methyl;    R1,R3=aliphatic unsaturated group;    R2,R4=aliphatic (un)saturated monovalent hydrocarbyl;    n,p,q=1 or more;    m=3 or more;and    R5=H or saturated monovalent hydrocarbyl.    When q is 1 or more, p is 2q or more.