The present invention is to provide an epoxy resin composition uniformly containing a large amount of inorganic fillers, excellent in heat resistance and flame resistance, and having good impregnation into a base material, and a prepreg using the epoxy resin composition, having good tackiness, and being easy in handling. Furthermore, it is to provide a printed wiring board using a metal-clad laminate formed using the prepreg and/or the prepreg or the epoxy resin composition, capable of easily conducting an ENEPIG process, and a semiconductor device using the printed wiring board, excellent in performances. An epoxy resin composition comprises a solid epoxy resin, a silica nanoparticle having an average particle diameter of 1 nm or more and 100 nm or less, and a silica particle having an average particle diameter larger than that of the silica nanoparticle, in the range of 0.1 μm or more and 5.0 μm or less.