The purpose of the present invention is to provide a silicone-modified epoxy resin which produces a cured product having excellent low gas permeability and strength; a composition of the resin; and an epoxy resin cured product obtainable by curing the composition.Disclosed is an epoxy resin represented by the following Formula (1): wherein R1 independently represents a monovalent aromatic hydrocarbon group having 6 to 12 carbon atoms; R2 independently represents a monovalent aliphatic hydrocarbon group having 1 to 12 carbon atoms; R3 independently represents a monovalent aliphatic hydrocarbon group having 1 to 12 carbon atoms or a monovalent aromatic hydrocarbon group having 6 to 12 carbon atoms; R4 represents an oxygen atom or a divalent hydrocarbon group having an aliphatic cyclic structure; R5 represents silicone chain having a norbornane epoxy structure at either end; and X represents an organic group having a norbornane epoxy group.