A resin which comprises (1) at least one structural unit selected from the group consisting of a structural unit derived from 3-hydroxy-1-adamantyl (meth)acrylate, a structural unit derived from 3,5-dihydroxy-1-adamantyl (meth)acrylate, a structural unit derived from (meth)acryloyloxy-.gamma.-butyrolactone having a lactone ring optionally substituted by alkyl, a structural unit of the formula (Ia) and a structural unit of the formula ##STR00001## and (2) a structural unit of the formula (II) ##STR00002## and which itself is insoluble or poorly soluble in an alkali aqueous solution but becomes soluble in an alkali aqueous solution by the action of an acid; and also provides a chemical amplification type positive resist composition comprising a resin defined above and an acid generator.