Antimicrobial agents are used to treat bacterial diseases in Japanese aquaculture. Although the number of approved antimicrobial agents has varied over time, currently 13 drugs are used. While antimicrobial agents have been effective in treating bacterial diseases in cultured fish, many antimicrobial-resistant bacteria have been reported. The problem of drug-resistant bacteria is important not only as a disease control measure for farmed fish, but also because of its relationship to human pathogens. In Japan, the "National Action Plan on Antimicrobial Resistance (AMR)" has been formulated, and monitoring for antimicrobial resistance has been conducted as part of the plan. In antimicrobial susceptibility testing of Lactococcus garvieae conducted since 2004, resistance has emerged to only three drugs, erythromycin (EM), oxytetracycline (OTC), and lincomycin (LCM). In 2004, LCM resistance accounted for more than 90%, and the rate of EM and OTC resistance was approximately 50%. However, since 2013, EM and OTC resistance has almost disappeared, and the LCM resistance rate has decreased to approximately 60%. This is thought to be largely because of vaccines, but the importance of chemotherapy with antimicrobial agents has not changed, as antigenic variants have emerged that vaccines are ineffective against. It is theorized that antimicrobial-resistant bacteria and genes emerged through usage of antimicrobials in aquaculture that are associated with antimicrobial resistance in human pathogens. In the future, even more prudent use of antibacterial agents will be required.