A positive resist composition comprises: (A) a compound that generates an acid upon irradiation with an actinic ray or radiation; and (B) a resin that has an acid-decomposable repeating unit represented by formula (I'), has a dispersity of 1.5 or less and increases its solubility in an alkali developer by action of an acid, ##STR00001## wherein Xa.sub.1 represents a hydrogen atom, an alkyl group, a cyano group or a halogen atom; Ry.sub.1 to Ry.sub.3 each independently represents an alkyl group or a cycloalkyl group, and at least two members out of Ry.sub.1 to Ry.sub.3 may combine to form a ring structure; and Z represents a divalent linking group.