A positive resist composition with a broad DOF and a method for resist pattern formation are provided. This composition is a positive resist composition which includes a resin component (A) that exhibits increased alkali solubility under the action of acid and an acid generator component (B) that generates acid upon exposure, wherein the component (A) is a copolymer that contains n [wherein, n is an integer from 4 to 6] structural units with mutually different structures, and the proportion of each structural unit within the copolymer is greater than 0 mol % but no higher than 100/(n−1) mol %.