Abstract: A resin composition for encapsulating an optical semiconductor element that comprises components (A), (B) and (C) described below: (A) a branched silicone resin containing 2 to 6 epoxy groups, one or more (R1SiO3/2) units, two or more (R2R3SiO)n structures and 3 or more (R43-xR5xSiO1/2) units per molecule, in an amount of 100 parts by weight, wherein R1, R2, R3, R4 and R5 each represents a hydrogen atom, a hydroxyl group or a monovalent organic group of 1 to 20 carbon atoms which monovalent organic group may or may not contain an oxygen atom, provided that at least two of the R5 groups within each molecule represent an epoxy group and/or an epoxy group-containing non-aromatic group, n represents an integer of 3 to 20, and x represents an integer of 1 to 3, (B) a curing agent, in such an amount that a content of epoxy reactive groups in the component (B) ranges from 0.4 to 1.5 moles per 1 mol of epoxy groups within component (A), and (C) a curing catalyst, in an amount within a range from 0.