A method of preparing an antibacterial polymer and application of the antibacterial polymer obtained by this method. A silver oxide and a thermoplastic copolymer that contains a unit consisting of ethylenically unsaturated carboxylic acid or an anhydride thereof and that can be melted upon heating, are mixed, melted and is kneaded in order to obtain a polymer that exhibits in combination the antibacterial property and the heat-processability or the heat-moldability. By making this polymer present on the inner surface of the packing containers, it is made possible to impart the antibacterial property without deteriorating the flavor retentivity for the contents.