To provide an antimicrobial agent high in safety and bactericidal action against pathogenic microbes, showing sustainable bactericidal action and no deterioration in bactericidal action during a process of sterilization of meat or the like, and repeatedly used for a sterilization process.

This antimicrobial agent derived from a naturally occurring substance contains polar solvent-extracted leaves of an Eucalyptus plant, chitosan and polylysine. It shows higher effect when it contains the extract and chitosan in a wt. ratio of (1:10) to (10:1), and polylysine at 0.1 to 10 pts.wt. per pt.wt. of the extract and chitosan totaled. The polar solvent is preferably selected from the group consisting of lower alcohols and glycols.