To provide a method for producing a polymer by which the polymer having a narrow molecular weight distribution can efficiently be produced even by a cationic polymerization.

The method for producing the polymer includes introducing a cationic polymerization initiator and a first cationically polymerizable monomer to the first joining part by each flowing method, mixing the cationic polymerization initiator with the first cationically polymerizable monomer at the joining part, and polymerizing the first cationically polymerizable monomer in the first reaction tube connected to the first joining part. The method is regulated so that the inner diameter X (mm) in the first joining part may be within the range of 0.01-10 mm and the flow rate Y (mL/min) of the liquid in the first reaction tube may satisfy the relation of expression (1): (20X)Y (1).