There are provided a complex of lactoferrin with a hydrophilic, linear and non-peptide polymer which has a prolonged life in vivo and a high clinical usefulness due to its reduced antigenicity and imparted pepsin-resistance, and a method of producing the same. There are also provided a lactoferrin complex which sustains the biological activity of natural lactoferrin at a certain ratio, shows a significantly prolonged life in vivo and is superior in clinical usefulness to natural lactoferrin, a method of producing the same and the like. One aspect of the invention relates to a method of producing a polyethylene glycolated lactoferrin complex wherein linear polyethylene glycol (PEG) or a modified product thereof is covalently bonded to lactoferrin via an amide bond, the method including causing a reaction in a reaction liquid, which contains lactoferrin and a linear PEG derivative having a para-nitrophenyl group, under conditions of allowing formation of an amide group between the para-nitrophenyl group and lactoferrin.
