NOVELTY - An addition-curable silicone resin composition contains organosilicon compound comprising organopolysiloxane (I), organohydrogenpolysiloxane having 2 or more hydrogen atoms coupled with silicon atom in a molecule, and platinum-type catalyst having platinum content of 0.1-5 ppm. The organosilicon compound (30-100 %mass) has 2 or more noncovalent double bond groups in a molecule.    USE - Addition-curable silicone resin composition is used for sealing optical-semiconductor element in optical semiconductor device (both claimed) e.g. light-emitting-diode-type light-emitting device. The optical-semiconductor element is phototransistor, photodiode, charge coupled device, solar cell module, photocoupler and erasable programmable read only memory.    ADVANTAGE - The addition-curable silicone resin composition has excellent sulfide resistance. The optical semiconductor device containing the addition-curable silicone resin composition has excellent reliability.    DETAILED DESCRIPTION - An addition-curable silicone resin composition contains organosilicon compound comprising organopolysiloxane of formula: (R1SiO3/2)a(R2R3SiO)b(R4R5R6SiO1/2)c(SiO4/2)d (I), organohydrogenpolysiloxane having 2 or more hydrogen atoms coupled with silicon atom in a molecule, and platinum-type catalyst having platinum content of 0.1-5 ppm. The organosilicon compound (30-100 %mass) has 2 or more noncovalent double bond groups in a molecule.    R1-R6=homogeneous or heterogeneous monovalent hydrocarbyl, in which monovalent hydrocarbyl (1-50 mol%) is noncovalent double bond-containing group;    a-d=positive numbers;    a/(a+b+c+d)=0-0.95;    b/(a+b+c+d)=0.05-1;    c/(a+b+c+d)=0-0.05;    d/(a+b+c+d)=0-0.10;and    a+b+c+d=1.    An INDEPENDENT CLAIM is included for optical semiconductor device.    DESCRIPTION OF DRAWING(S) - The drawing shows the sectional view of the light-emitting-diode-type light-emitting device.    Light-emitting-diode chip (1)    Electroconductive wire (2)    Package (3)    Silicone resin portion (4)