A resin composition for sealing an optical semiconductor, the resin composition comprising a plurality of aromatic compounds satisfying the following relationship 1:
- (A1×A2)/A3+(B1×B2)/B3+ . . . <0.0005 (1)
- wherein A1 represents a mass ratio of an aromatic compound, A2 represents a number of aromatic rings per molecule of the aromatic compound, and A3 represents a molecular weight of the aromatic compound;
- and A, B, . . . each represent an aromatic compound,
- wherein the resin composition comprises an epoxy, an alicyclic acid anhydride, and an antioxidant, and
- wherein the resin composition has a linear transmittance at a wavelength of 300 nm of 80% or higher and a linear transmittance at a wavelength of 400 nm of 95% or higher when formed into a cured product by being heated and molded at 150° C. for four minutes and then heated at 150° C. for three hours, the cured product having a size of 50 mm in width, 50 mm in length, and 1 mm in thickness.