The invention provides an epoxy resin composition, a novel epoxy resin, a novel phenol resin and a semiconductor encapsulating material. The cured article of the epoxy resin composition has superior characteristics in flame retardancy, heat-resistant and curing. The epoxy resin composition is a preferable resin composition using in a semiconductor device or a circuit board device, and has the structure in which a naphthalene structure is bonded with an arylene group through an oxygen atom, and the total number of the aromatic nucleus in both the naphthalene structures and the arylenes group is 2-8. The epoxy resin composition essentially includes an epoxy resin (A) having the glycidoxy group as a substituent in the aromatic nucleus and a curing agent (B).