To provide an N-vinyllactam-based polymer that has little change (yellowing) in color tone and can maintain color tone, even under the condition of high temperatures.

In a method for manufacturing a polymer having a structural unit derived from an N-vinyllactam-based monomer, this method for manufacturing an N-vinyllactam-based polymer includes a step of polymerizing a monomer component which contains an N-vinyllactam-based monomer, in an aqueous solvent in the presence of an azo-based polymerization initiator, and/or a water-soluble organic peroxide, and also a reducing agent.