A 78-year-old man was admitted to the hospital because of pneumonia on December 16, 2002. As pneumonia did not improve, intravenous infusion of PZFX, a new quinolone antimicrobial agent, was started at a dose of 1000mg/day on December 21, 2002. Eight hours after administration of PZFX, general fatigue, dyspnea and oliguria were noted. Urinary output further decreased and the patient developed acute renal failure on the next day. Serum enzymes, including GOT, GPT, LDH, CPK and serum aldolase and urine myoglobin were markedly elevated, and rhabdomyolysis due to PZFX was diagnosed. Hemodialysis was started on the same day and high levels of serum enzymes promptly decreased. Chest X-ray revealed abdominal free air beneath the bilateral diaphragm on January 8, 2003. However, his symptoms and laboratory data did not suggest severe peritonitis. Both gastroduodenoscopy and colonoscopy showed no abnormal findings. The diagnosis of idiopathic pneumoperitoneum was established. Abdominal free air disappeared in chest x-ray on January 15, 2003. This is the 19^th case of adult idiopathic pneumoperitoneum reported in the Japanese literature.