To provide a radiation-sensitive resin composition, particularly a copolymer useful as a resin component of the same, high in transparency to radiation; excellent in basic characteristics of a resist such as sensitivity, resolution, and pattern configuration; concurrently high in dry etching resistance and adhesion to the substrate; and excellent in developability and conservation stability.

The copolymer has a repeating unit derived from a monomer represented by 1-ethylcyclopentyl ester or by 2-ethyladamantan-2-yl ester, etc., of a carboxy group-containing norbornene compound; and another repeating unit derived from a lactone monomer represented by 2(5H)-furanone or by its derivative, etc. In the copolymer, the content of the former repeating unit accounts for more than 30 mol% of the repeating units in total. The radiation- sensitive resin composition contains the copolymer and a radiation-sensitive acid generator.