Positive resist composition and method of forming resist pattern

A positive resist composition comprising:

• a resin (A) that increases alkali solubility of the resin due to action of an acid, wherein the resin comprises a copolymer containing a first structural unit (a1) derived from hydroxystyrene, and a second structural unit (a2) derived from a (meth)acrylate with an alcoholic hydroxyl group, and a weight average molecular weight of the copolymer is within a range from 2000 to 8500, and 10 mol% to 25 mol% of a combined total of hydroxyl groups within the structural units (a1) and alcoholic hydroxyl groups within the structural units (a2) are protected with acid dissociable, dissolution inhibiting groups;
• an acid generator (B) that generates an acid on exposure to light;
• and polypropylene glycol (C), provided that the hydroxyl groups within (a1) and the alcoholic hydroxyl groups within (a2) are both partially protected by the acid dissociable, dissolution inhibiting groups.