While high-performance semiconductor devices encapsulated with a resin composition are subject to the problem of wrong operation due to the alpha-particles emitted from the trace amounts of radioactive impurities, e.g. uranium and thorium, contained in the silica filler incorporated in the resin composition, a means for solving this problem is provided by use of a silicon dioxide powder obtained by the pyrolysis of a volatilizable silicon compound free from radioactive impurities in an oxidizing condition and having an average particle diameter in the range from 0.5 to 100 .mu.m in place of the conventional silica fillers.