Provided are a resin composition capable of yielding a cured product exhibiting a superior heat resistance over a long period of time, and a superior reflow resistance due to a low water absorption rate even at a high temperature; and a semiconductor device encapsulated by such cured product. The resin composition is a heat-curable resin composition containing: (A) an organopolysiloxane having, in one molecule, at least one cyclic imide group and at least one siloxane bond; (B) an inorganic filler; and (C) a radical polymerization initiator. The semiconductor device is encapsulated by the cured product of such heat-curable resin composition.