The present invention provides a polymer which has at least one or more of a repeating unit represented by a following general formula (1a), a repeating unit represented by a following general formula (2a) and a repeating unit represented by a following general formula (3b), and a repeating unit represented by a following general formula (1c), and a positive resist composition which contains as a base resin the polymer. Thereby, there can be provided a positive-resist composition having high sensitivity and high resolution in exposure with a high energy beam, wherein line edge roughness is small since swelling at the time of development is suppressed, and the residue after development is few ##STR00001##