PURPOSE: To obtain an epoxy resin having rapid thermal curability and photocurability, excellent in adhesivity, and suitable for coatings, adhesives, laminates, molding materials, etc., by reacting a polybasic acid with an epoxy resin in a specific ratio.

CONSTITUTION: This epoxy resin containing radically polymerizable unsaturated groups is produced by reacting (A) a polybasic acid containing an unsaturated dibasic acid such as maleic acid, fumaric acid, itaconic acid or citraconic acid with (B) an epoxy resin having two or more epoxy groups in the molecule (e.g. a bisphenol A, F or S type epoxy resin) in such a ratio that the carboxyl groups (Ac: mole number) of the component A and the epoxy groups (Be: mole number) of the component B satisfy a relation of inequality: 0.1≤Ac/Be<1.0.