PROBLEM TO BE SOLVED: To provide an antibacterial agent that has more excellent antibacterial performance than conventional antibacterial agents.SOLUTION: Provided is an antibacterial agent containing a copolymer having cationic groups. The copolymer comprises a structural unit derived from a cationic group-containing monomer and a structural unit derived from a hydrophobic monomer, 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 structural units, and has a weight average molecular weight of 4,000 to 1,000,000. The hydrophobic monomer has, as a homopolymer, a solubility parameter of 15 or less.SELECTED DRAWING: None
