An optical semiconductor device in which an optical semiconductor element connected to a silver-plated copper lead frame is sealed with a cured product of an addition curing silicone resin composition, wherein the addition curing silicone resin composition comprises: organopolysiloxane that contains an aryl group and an alkenyl group and does not contain an epoxy group; organohydrogenpolysiloxane that has at least two hydrosilyl groups (SiH groups) per molecule and also has an aryl group, the organohydrogenpolysiloxane that contains 30 mol% or more of an HR 2 SiO 0.5 unit (R represents the same or different monovalent hydrocarbon groups which contain no aliphatic unsaturated bond) in a constituent unit having an amount that a molar ratio of the hydrosilyl group in the component with respect to the alkenyl group in the component (SiH group/alkenyl group) is 0.70 to 1.00; and a hydrosilylation catalyst having a catalytic amount, wherein the component is a mixture of a silicone resin represented by the following average composition formula and a silicone oil represented by the following average composition formula, a blending ratio in terms of mass ratio of the silicone resin: the silicone oil being 90 to 50: 10 to 50, and the component is organohydrogenpolysiloxane represented by the following formula and, R 1 a (R 2) b (R 3) c SiO (4-a-b-c)/2 wherein R 1 represents an aryl group having a carbon number of 6 to 14, R 2 represents a substituted or unsubstituted univalent hydrocarbon group having a carbon number of 1 to 10 excluding the aryl group and the alkenyl group, R 3 represents an alkenyl group having a carbon number of 2 to 8, a is 0.3 to 1.0, b is 0.05 to 1.5, c is 0.05 to 0.8, and a+b+c=0.5 to 2.0, wherein R 4 represents a group selected from R 1, R 2, and R 3 which may be equal to or different from each other, and at least 5 mol% corresponds to the aryl group, g represents an integer that is 1, 2, or 3, and x represents an integer that is not smaller than 1 with which a degree of viscosity at 25°C is 10 to 1,000,000 mPa-s, wherein R 6 represents a monovalent hydrocarbon group having a carbon number of 1 to 10, R 7 represents a monovalent hydrocarbon group containing at least one aryl group, and m represents an integer of 1 to 4, wherein R 6 and R 7 are equal to those described above, and n represents an integer of 1 to 100. organopolysiloxane that contains an aryl group and an alkenyl group and does not contain an epoxy group; organohydrogenpolysiloxane that has at least two hydrosilyl groups (SiH groups) per molecule and also has an aryl group, the organohydrogenpolysiloxane that contains 30 mol% or more of an HR 2 SiO 0.5 unit (R represents the same or different monovalent hydrocarbon groups which contain no aliphatic unsaturated bond) in a constituent unit having an amount that a molar ratio of the hydrosilyl group in the component with respect to the alkenyl group in the component (SiH group/alkenyl group) is 0.70 to 1.00; and a hydrosilylation catalyst having a catalytic amount, the component is organohydrogenpolysiloxane represented by the following formula and, R 1 a (R 2) b (R 3) c SiO (4-a-b-c)/2 wherein R 1 represents an aryl group having a carbon number of 6 to 14, R 2 represents a substituted or unsubstituted univalent hydrocarbon group having a carbon number of 1 to 10 excluding the aryl group and the alkenyl group, R 3 represents an alkenyl group having a carbon number of 2 to 8, a is 0.3 to 1.0, b is 0.05 to 1.5, c is 0.05 to 0.8, and a+b+c=0.5 to 2.0, wherein R 4 represents a group selected from R 1, R 2, and R 3 which may be equal to or different from each other, and at least 5 mol% corresponds to the aryl group, g represents an integer that is 1, 2, or 3, and x represents an integer that is not smaller than 1 with which a degree of viscosity at 25°C is 10 to 1,000,000 mPa-s, wherein R 6 represents a monovalent hydrocarbon group having a carbon number of 1 to 10, R 7 represents a monovalent hydrocarbon group containing at least one aryl group, and m represents an integer of 1 to 4, wherein R 6 and R 7 are equal to those described above, and n represents an integer of 1 to 100.