An epoxy resin composition for encapsulating an optical semiconductor element, which has small internal stress and also can obtain good light transmittance within a broad temperature range. An epoxy resin composition for encapsulating an optical semiconductor element comprising the following component (A):(A) an epoxy resin complex which comprises an epoxy resin as the matrix component and silicon dioxide particles (a) dispersed therein:(a) silicon dioxide particles having an average particle size of from 5 to 40 nm measured by the small angle neutron scattering (SANS).