Provided are a polymer that can be favorably used as a positive resist having a low film reduction rate in a state of low irradiation with ionizing radiation or the like and a positive resist composition that can favorably form a high-resolution pattern. The polymer includes an α-methylstyrene unit and a methyl α-chloroacrylate unit, and the proportion of components having a molecular weight of less than 6,000 in the polymer is no greater than 0.5%. The positive resist composition contains the aforementioned polymer and a solvent.