PROBLEM TO BE SOLVED: To provide an antibacterial agent exhibiting good antibacterial performance. An antibacterial agent containing a cationic group-containing copolymer, wherein the copolymer has a structural unit (a) derived from a cationic group-containing monomer represented by the following general formula and a hydrophobic simple substance. It has a structural unit (b) derived from a weight, the copolymer contains 36 to 99.9% by mass of the structural unit (a), and the weight average molecular weight of the copolymer is 4000 to 1,000,000. , The hydrophobic monomer is an antibacterial agent having a solubility parameter of a copolymer of 15 or less. In the formula, R1 represents a hydrogen atom or a methyl group, R2, R3 and R4 independently represent a hydrogen atom or an alkyl group having 1 to 24 carbon atoms, X represents a divalent linking group, and Y− represents a divalent linking group. Represents an anion species. [Selection diagram] None【課題】良好な抗菌性能を示す抗菌剤の提供。【解決手段】カチオン性基含有共重合体を含む抗菌剤であって、該共重合体は、下記一般式で表されるカチオン性基含有単量体由来の構造単位（ａ）と疎水性単量体由来の構造単位（ｂ）とを有し、該共重合体は、構造単位（ａ）を３６～９９．９質量％含み、該共重合体の重量平均分子量は４０００～１００万であり、該疎水性単量体は、単独重合体の溶解性パラメータが１５以下である抗菌剤である。式中、Ｒ１は水素原子又はメチル基を表し、Ｒ２、Ｒ３及びＲ４はそれぞれ独立して水素原子又は炭素数１～２４のアルキル基を表し、Ｘは２価の連結基を表し、Ｙ－はアニオン種を表す。【選択図】なし
