Provided are a polymer that can be favorably used as a positive resist having a low film reduction rate under low irradiation, a high γ value, and high sensitivity, and a positive resist composition that can efficiently form a high-resolution pattern. The polymer includes an α-methylstyrene unit and a methyl α-chloroacrylate unit, and has a molecular weight distribution (Mw/Mn) of less than 1.48. In the polymer, the proportion of components having a molecular weight of less than 6,000 is no greater than 0.5% and the proportion of components having a molecular weight of greater than 80,000 is no greater than 6.0%. The positive resist composition contains the aforementioned polymer and a solvent.