To find out a vinyl ester precisely developable and capable of forming a cured film having excellent flexibility and heat resistance, and also to provide a radically polymerizable resin composition applicable to a flexible printed wiring board, etc.

An acid-modified vinyl ester having an acid value of 30-150 mg KOH/g, synthesized from a bisphenol type epoxy resin and/or its bromide, long-chained polybasic acid, unsaturated monobasic acid and/or glycidyl- containing unsaturated monomer and a polybasic acid anhydride, and the radically polymerizable resin composition containing the acid-modified vinyl resin are provided.