Positive resist composition, positive resist composition for thermal flow, and resist pattern forming method

A positive resist composition comprising a resin component (A) which exhibits increased alkali solubility under an action of an acid, and an acid generator component (B) which generates an acid upon exposure, wherein

• the resin component (A) contains a polymer compound (A1) containing a structural unit (a0) represented by general formula (a0) shown below:
• (wherein R represents a hydrogen atom, a halogen atom, a lower alkyl group, or a halogenated lower alkyl group;
• Y1 represents an aliphatic cyclic group;
• Z represents an acid dissociable, dissolution inhibiting group containing a tertiary alkyl group;
• a represents an integer from 1 to 3, b represents an integer from 0 to 2, and a+b=1 to 3;
• c, d and e each represents, independently, an integer from 0 to 3), and a structural unit (a2) derived from an acrylate ester containing a lactone-containing cyclic group.