A silicone-modified epoxy resin which yields a cured product having low gas permeability and excellent strength; a composition of the silicone-modified epoxy resin; and an epoxy resin cured product obtainable by curing the composition, are provided.An epoxy resin represented by the following Formula (1): wherein R1 represents a hydrocarbon group having 1 to 6 carbon atoms; X represents an organic group having a norbornane epoxy structure, or a hydrocarbon group having 1 to 6 carbon atoms; n represents an integer from 1 to 3; plural R1s and plural Xs present in the formula may be respectively identical or different; and two or more of plural Xs represent an organic group having a norbornane epoxy structure.