The usefulness of switch therapy, from injection to oral medicine, for the treatment of peritonitis was evaluated. Thirty-five patients, who agreed to enroll the study, were randomly assigned to four treatment groups; one group treated with carbapenem antibacterial agent alone and three groups treated with switch therapy, in which injectable quinolone was switched to oral quinolone. For the intravenous administration group, if the patient showed the tendency of improvement by the third day, the intravenous injection was continued. However, if the patient did not show any improvement, the medication was changed to other medicine. For the switch therapy group, if the body temperature dropped to 37.5 degrees C or lower for at least 8 hours and if blood findings and clinical findings showed the tendency of improvement by the fourth day, the medication was switched to oral medicine. There was no difference in therapeutic effects among treatment groups. However, both duration of hospitalization and total medical costs were significantly reduced in the switch therapy groups comparing to those in the intravenous administration group. The results of this study showed that the switch therapy, from injection to oral medicine, was one of useful treatments in treating peritonitis.