Positive resist composition and method of forming resist pattern

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

• said resin component (A) comprising:
• a polymer (A1) comprising a structural unit (a0) represented by general formula (a0) shown below and no structural unit (a1) derived from an acrylate ester containing an acetal-type acid dissociable, dissolution inhibiting group, exclusive of said structural unit (a0), and
• a polymer (A2) comprising said structural unit (a1) and no structural unit (a0):
• 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 a tertiary alkyl group-containing group or an alkoxyalkyl group;
• a represents an integer of 1 to 3, and b represents an integer of 0 to 2, with the proviso that a+b=1 to 3;
• and each of c, d and e independently represents an integer of 0 to 3.