A method of manufacturing a castable epoxy resin composition which comprises as components an epoxy resin, an acid anhydride in an amount sufficient for hardening, an inorganic filler, and acrylic rubber particles having a thermoplastic resin skin, said thermoplastic resin skin being 1 to 50 wt % of said acrylic rubber particles, wherein each of said acrylic rubber particles has epoxy groups on a surface thereof, and said acrylic rubber particles are present in an amount ranging between 2 and 40 parts by weight based on 100 parts by weight of said epoxy resin, wherein said inorganic filler is an alumina powder having a mean particle size by volume of 6 to 16 .mu.m, and a particle size of at least 10 volume % of said alumina powder particles is 20 .mu.m or greater, and wherein at least 90 volume % of said rubber particles are dispersed as discrete particles with a particle size by volume of 1 .mu.m or less, said method comprising the steps of: preparing a modified hardener by mixing said acrylic rubber particles with at least a portion of said acid anhydride by using shearing force applying means in sufficient force to uniformly disperse said rubber particles in said acid anhydride; and mixing said modified hardener and the remainder of said components.