Disclosed is a method for the preparation of a complex in which a physiologically active polypeptide is covalently bonded to an immunoglobulin constant region via a non-peptidyl linker. The method is characterized by the employment of a reducing agent, by which conventional problems of low production yield and modification of the polypeptide can be overcome. The physiologically active polypeptide-non-peptidyl polymer-immunoglobulin constant region complex can be produced with high purity and yield as well as at low cost. Thus, the method is industrially useful. Moreover, by exhibiting a prolonged action profile, the physiologically active polypeptide-non-peptidyl polymer-immunoglobulin constant region complex can be effectively used for developing long-acting formulations of physiologically active polypeptides which have improved drug compliance.
