Provided are a positive resist composition and a resist pattern forming method having fewer defects and superior lithographic characteristics. The positive resist composition includes a resin component (A) which has on a main chain a structural unit derived from an (α-lower alkyl)acrylate ester and exhibits increased alkali solubility under the action of an acid, and an acid generating component (B) which generates the acid upon irradiation with radiation, in which the resin component (A) is a copolymer having at least two structural units which is obtained by incorporating an acid when polymerizing at least one monomer for the production thereof.