Silicone resin, sealing material, and optical semiconductor device

An optical semiconductor device including

• an optical semiconductor element, and
• a sealing material comprising a silicone resin, the sealing material sealing in the optical semiconductor element
• wherein the silicone resin is obtained by allowing
• a cage octasilsesquioxane having a group represented by formula (1) below to reach with
• an alkenyl group-containing polysiloxane containing an alkenyl group having a molarity smaller than the molarity of a hydrosilyl group of the cage octasilsesquioxane
• in the presence of a hydrosilylation catalyst,
• such that a hydrosilyl group of the cage octasilsesquioxane remains in a ratio at which a molar ratio of an alkenyl group of the alkenyl group-containing polysiloxane to the hydrosilyl group of the cage octasilsesquioxane is below 1:
• (where R1 represents a monovalent hydrocarbon group, R2 represents hydrogen or a monovalent hydrocarbon group, and the molar ratio of monovalent hydrocarbon group:
• hydrogen in R2 in the cage octasilsesquioxane as a whole is, as an average value, in a range of 6.5:1.5 to 5.5:2.5).