With the introduction of the diagnosis procedure combination (DPC) system, the reduction of the health expenditure is expected. The use of generic drugs is promoted. It is imperative for introduction of generic drugs to assure the appropriate use of drugs to decrease medical costs without reducing medical services. Additionally, we should make a concerted effort to prevent the injudicious use of antimicrobial agents resulting in the emergence of drug resistance. In this study, we assessed the effects of switching to generic antimicrobial drugs on drug purchases cost, AUD (antimicrobial use density), hospital days, and antimicrobial susceptibility of Pseudomonas aeruginosa. This study was carried out at Yashima General Hospital for the period from April 2008 to September 2011. The Health, Labor and Welfare Ministry designated Yashima General Hospital as a DPC hospital from July 2009. Since the DPC introduction, the drug purchase cost has been decreased about 40% without reduction of AUD, hospital days, and antimicrobial susceptibility of Pseudomonas aeruginosa. Hospitals are making efforts to assess and improve management efficiency while maintaining the quality of medical care. Our results suggested that the introduction of generic drugs with assessment of efficacy may be one of the useful methods for pharmacy management.