Curable copolymer and curable resin composition

A copolymer includes at least monomeric units (A) corresponding to a carbonyl- or acid-anhydride-group-containing polymerizable unsaturated compound (a) ; monomeric units (B) corresponding to at least one polymerizable unsaturated compound (b) selected from compounds represented by following Formulae (1) and (2) (wherein R a represents a hydrogen atom or, e.g., an alkyl group having 1 to 4 carbon atoms; and R b represents a single bond or, e.g., an alkylene group having 1 to 18 carbon atoms); and monomeric units (C) corresponding to at least one polymerizable unsaturated compound (c) selected from compounds represented by following Formulae (3) and (4) (wherein R c represents a hydrogen atom or, e.g., an alkyl group having 1 to 4 carbon atoms; and R d represents a single bond or, e.g., an alkylene group having 1 to 18 carbon atoms). The copolymer, when used as a radiation-sensitive resin, can give a high-performance cured film excellent in hardness, solvent resistance, and thermal stability.