An optical semiconductor device in which an optical semiconductor element connected to a silver-plated copper lead frame is sealed with an addition curing silicone resin composition, the addition curing silicone resin composition having (A) organopolysiloxane that contains an aryl group and an alkenyl group and does not contain an epoxy group; (B) organohydrogenpolysiloxane that has at least two hydrosilyl groups per molecule and an aryl group, the organohydrogenpolysiloxane that contains 30 mol % or more of an HR2SiO0.5 unit in a constituent unit having an amount that a molar ratio of the hydrosilyl group in the component (B) with respect to the alkenyl group in the component (A) is 0.70 to 1.00; and (C) a hydrosilylation catalyst having a catalytic amount.