An epoxy resin composition for encapsulating optical semiconductor element, comprising the following (A) to (D): (A) an epoxy resin component comprising a novolak type epoxy resin having at least one of the biphenyl skeleton and naphthalene skeleton in one molecule in an amount of from 50 to 100% by weight of the total weight of the epoxy resin component, (B) a curing agent component comprising a novolak type phenol resin having at least one of the biphenyl skeleton and naphthalene skeleton in one molecule in an amount of from 50 to 100% by weight of the total weight of the curing agent component, (C) an organic phosphorus flame retarder, and (D) a curing catalyst, wherein the content of an organic phosphorus flame retarder as (C) is from 1.5 to 10% by weight based on the total weight of the epoxy resin composition.