To provide a method for producing a polymer, capable of reducing the amount of a colored substance formed in the production or after the production of the polymer having amino groups to produce the polymer having a good color.

This method for producing the polymer is provided by polymerizing a monomer components containing the following monomer (A) and monomer (B) and neutralizing a part of the monomer (A) before the polymerization reaction or in the polymerization reaction with an organic acid without containing an aromatic ring. The fiber surface-modifying polymer obtained by the method is also provided. Provided that, (A) an unsaturated bond-containing monomer having at least 1 kind of an amino group selected from primary to tertiary amino groups, (B) an unsaturated bond-containing monomer having a hydrophobic group, and without having the primary to tertiary amino groups.