An epoxy resin composition includes (A) an epoxy resin, (B) a curing agent, and (C) an epoxidized polyisoprene (c-1) that contains an epoxy group at 0.15 to 2 meq/g in the molecule and has a number-average molecular weight of 15000 to 200000 or an epoxidized polybutadiene (c-2) that contains an epoxy group at 0.15 to 2 meq/g in the molecule and has a number-average molecular weight of 20000 to 200000. The epoxy resin composition has high heat resistance and reduced internal stress, and can be suitably used in applications such as electronic part materials represented by semiconductor encapsulating materials and adhesives.