Positive resist composition and method of forming resist pattern

10″3−10 7979A positive resist composition including a resin component (A) and an acid-generator component (B), the resin component (A) including a structural unit (a1) derived from hydroxystyrene, and a structural unit (a2) having an acetal-type acid dissociable dissolution inhibiting group, and the acid-generator component (B) including an acid generator (B1-i) having at least one anion moiety selected from the group consisting of anion moieties represented by general formula (b-3), (b-4), and (b-5), an acid generator (B1-ii) having an anion moiety represented by general formula (b-6) shown below, or an acid generator (B1-iii) having a cation moiety represented by general formula (b′-3) shown below: wherein X″ represents an alkylene group of 2 to 6 carbon atoms in which at least one hydrogen atom has been substituted with a fluorine atom; Y″ and Z″, U″, V″, and W″ each independently represents an alkyl group of 1 to 10 carbon atoms in which at least one hydrogen atom has been substituted with a fluorine atom, [Chemical Formula 2] R-SO (b-6) wherein R″ represents a hydrocarbon group which may or may not have a substituent, wherein R″ to R″ each independently represents a phenyl group or naphthyl group that may or may not have a substituent; with the proviso that the case where all of R″ to R″ represent phenyl groups which do not have a substituent is excluded.