Disclosed is an additive for imparting antimicrobial properties to a resin. The additive comprises a string polymer, at least one terminal of which has a positive charge, wherein the string polymer is a polymer containing structural units derived from a cationic polymerization initiator and a monomer containing a carbon-carbon double bond. By using this additive, it becomes possible to obtain an antibacterial resin molded body which can be prepared simply and inexpensively and which has high stability and safety.