Resin compositions containing a setting resin and a low-molecular-weight novolak resin do not possess satisfactory heat resistance and mechanical strength characteristics for all intended uses. These difficulties can be overcome by employing a resin composition comprising (A) a setting resin and (B) 10 to 200 parts by weight, per 100 parts by weight of said resin (A), of a substantially linear, high-molecular-weight novolak substituted phenolic resin which is derived from a phenol component mainly comprising at least one bifunctional phenol and which has a number average molecular weight of at least 1500.