A thermosetting resin composition which is improved in impact resistance, thermal crack resistance, resistance to oxidative deterioration, and resistance to thermal deterioration while retaining intact heat resistance represented by HDT, and is suitable for use in the encapsulation of semiconductors, etc. It comprises a thermosetting resin and a liquid epoxidized polybutene and optionally contains a hardener, wherein the liquid epoxidized polybutene mostly has a terminal epoxy structure and a backbone structure in which at least 80% of the repeating units have a specific chemical structure. The thermosetting resin composition, through a curing reaction, gives a cured article in which the resinous part is constituted of a continuous phase formed mainly from the thermosetting resin and, dispersed therein, a dispersed phase having a size on the order of micrometer formed mainly from the liquid epoxidized polybutene.