An antimicrobial activity enhancing agent for lactoferrin or lactoferrin hyrdolysate with milk-derived protein which has the properties below as the active ingredient, and a method for producing the antimicrobial activity enhancing agent characterized by including: (a) a step of contacting raw milk to a cation exchange resin to adsorb the protein in the milk into the cation exchange resin； (b) a step of subjecting the cation exchange resin to a cleaning treatment after the adsorption treatment and passing the resin through a dissolution medium to elute a protein fraction with an isoelectric point of 9.0 to 11.0； and (c) a step of membrane filtering the eluted protein fraction using an ultrafiltration membrane having a molecular weight cut off of 30 kDa to prepare a milk-derived protein having the following properties:(A) an effect of enhancing antimicrobial activity of lactoferrin or lactoferrin hyrdolysate,(B) a molecular weight of 30 kDa or less,and(C) an isoelectric point of 9.0 to 11.0.
