An optical semiconductor device includes a metal lead frame including first and second plate portions, an optical semiconductor element mounted on the metal lead frame, and a reflector provided around the optical semiconductor element. A material for the reflector is an epoxy resin composition containing: (A) an epoxy resin; (B) a curing agent; (C) a white pigment; (D) an inorganic filler; and (E) at least one of a carboxylic acid and water. Components (C) and (D) are present in a total proportion of 69 to 94 wt % based on the amount of the overall epoxy resin composition, and the component (E) is present in a proportion of 4 to 23 mol % based on the total amount of the components (B) and (E). The resin composition has a higher glass transition temperature, and is excellent in moldability and blocking resistance and substantially free from warpage.