There have not been sufficient clinical studies based on pharmacokinetics/pharmacodynamics (PK/PD) theory, on which many clinical doctors have recently focused. To consider the optimized administration method based on PK/PD theory for gatifloxacin (GFLX), which was one of the oral fluoroquinolone antibacterial, we influenzae investigated clinical efficacies and adverse events for pelvic inflammatory disease (PID) in giving GFLX daily 400 mg divided twice a day or four times a day. The number of leukocyte and the value of CRP were significantly reduced by chemotherapy in twice a day group, compared with four times a day group. We were able to measure the blood level in 4 cases. The AUC/MIC values for presumption causative bacteria (causative bacteria in both cases: Escherichia coli) in cured patients were 142.28 and 280.16, however, in therapy-failed patients, the AUC/MIC value to presumption causative bacterium were 4.10 (causative bacteria: Prevotella bivia) and 4.35 (causative bacteria: Pseudomonas aeruginosa). These results suggested the importance of the therapeutic method based on PK/PD theory.