Without the use of a third monomer, no copolymer practicable as a base polymer for a radiation-sensitive resin composition has been obtained by copolymerizing two ingredients, i.e., a carboxylated monomer and an epoxidized monomer. A carboxylated monomer is reacted with a specific nonpolymerizable compound, and this reaction mixture is then copolymerized with an epoxidized monomer. A radiation-sensitive resin composition and a thermosetting resin composition each containing the resultant copolymer have satisfactory storage stability and are useful as a spacer for liquid-crystal display panels, etc., a planarization film for TFT elements, and a protective film for color filters.