The present invention is a positive resist composition and a resist pattern forming method including a resin component (A) which has a polymer compound (A1) having a structural units (a1) including an acetal type acid dissociable, dissolution inhibiting group, a structural unit (a2) derived from an acrylate ester having a lactone-containing polycyclic group, and a structural unit (a3) derived from an acrylate ester having a polar group-containing aliphatic hydrocarbon group, and an acid generator component (B) having an onium salt-based acid generator (B1) having a cation portion represented by a general formula (b-1) shown below [wherein, R11 represents an alkyl group, an alkoxy group, a halogen atom or a hydroxyl group; R12 to R13 each represents, independently, an aryl group or the alkyl group that may have substituent group; n′ represents either 0 or an integer from 1 to 3].