To provide an extremely useful carboxyl group-containing epoxy(meth) acrylate oligomer giving a water-soluble neutralized product, having excellent hydrolysis resistance, a low color number and high sensitivity, and capable of being used as a raw material for various kinds of functional polymers giving cured products having excellent heat resistance.

This carboxyl group-containing epoxy(meth)acrylate oligomer having carboxylic acids at the molecular terminals comprises the adduct of (c) an epoxy compound having two or more oxirane rings in the molecule to (a) a carboxylic acid compound having one or more (meth)acryloyl groups and two or more carboxyl groups in the molecule or the mixture of (a) the carboxylic acid compound with (b) a carboxylic acid compound having two or more carboxyl groups.