PROBLEM TO BE SOLVED: To provide an antibacterial agent that has more excellent antibacterial performance than conventional antibacterial agents.SOLUTION: An antibacterial agent contains a water-soluble copolymer that contains a structural unit derived from a cationic group-containing monomer represented by formula (5-3) and a structural unit derived from a hydrophobic monomer with a homopolymer solubility parameter of 15 or less, where a proportion of the structural unit derived from a cationic group-containing monomer is 36 to 99.9 mass% relative to 100 mass% of all the structural units, and has a weight average molecular weight of 4,000 to 2,000,000. (R8-R10 independently represent H or a C1-5 alkyl group； R5-R7 independently represent a C1-3 hydrocarbon group； X is a direct bond, a C1-12 alkylene group, a carbonyloxy (poly) methylene group or a [(poly) methylene]oxy-2-hydroxypropylene group； Y- is an anion).SELECTED DRAWING: NoneCOPYRIGHT: (C)2018,JPO&INPIT【課題】従来の抗菌剤よりも抗菌性能に優れた抗菌剤の提供。【解決手段】式（５－３）で表されるカチオン性基含有単量体由来の構造単位と、単独重合体の溶解性パラメータが１５以下である疎水性単量体由来の構造単位とを有し、該カチオン性基含有単量体由来の構造単位の割合が、全構造単位１００質量％に対して３６～９９．９質量％であり、重量平均分子量が４０００～２０万である水溶性共重合体を含む抗菌剤。（Ｒ８～Ｒ１０は、各々独立に、Ｈ又はＣ１～５のアルキル基；Ｒ５～Ｒ７は、各々独立に、Ｃ１～３の炭化水素基；Ｘは、直接結合、Ｃ１～１２のアルキレン基、カルボニルオキシ（ポリ）メチレン基又は、［（ポリ）メチレン］オキシ－２－ヒドロキシプロピレン基；Ｙ－は、陰イオン）【選択図】なし
