A positive resist composition and method of forming a resist pattern are provided which enable formation of a resist pattern having excellent shape with reduced LWR.The positive resist composition includes a resin component (A) which exhibits increased alkali solubility under action of acid and an acid-generator component (B) which generates acid upon irradiation, the resin component (A) having a structural unit (a1) which has an acetal-type protection group, and the acid-generator component (B) including an acid generator (B1) having a cation moiety represented by general formula (b-5) shown below: wherein Ra and Rb each independently represents an alkyl group, an alkoxy group, a halogen atom or a hydroxyl group; Rc represents an aryl group or alkyl group which may or may not have a substituent; and n′ and n″ each independently represents an integer of 0 to 3.