Polymer compound, positive resist composition, and method of forming resist pattern

A positive resist composition, comprising a resin component (A) which displays increased solubility in an alkali developing solution under action of acid;

• and an acid generator component (B) which generates an acid upon exposure, wherein the resin component (A) comprises a polymer compound (A1) which comprises a structural unit (a0) represented by a general formula (a0-1) shown below, and a structural unit (a1) derived from an acrylate ester containing an acid dissociable, dissolution inhibiting group:
• (wherein R represents a hydrogen atom, a lower alkyl group, or a halogenated lower alkyl group;
• two of R′ each independently represents a hydrogen atom, a lower alkyl group, or an alkoxy group of 1 to 5 carbon atoms;
• and X represents an alkylene group of 1 to 5 carbon atoms, an oxygen atom, or a sulfur atom.), and
• wherein the proportion of the structural unit (a0) in the polymer compound (A1) is from 5 to 60 mol %.