A 40's-year-old woman who had abdominal pain with fever was referred to our hospital for further examinations. Abdominal computed tomography showed no focal lesion, and no causative lesion was found after a gynecological examination, upper gastrointestinal endoscopy and colonoscopy. Tuberculin test and QuantiFERON-TB were positive, and thus tuberculous peritonitis was suspected. The level of adenosine deaminase (ADA) in ascites was high, and (18)F-fluorodeoxyglucose-positron emission tomography (FDG-PET) showed that FDG accumulated diffusely along the peritoneum. These findings supported the findings of tuberculous peritonitis. Final diagnosis of tuberculous peritonitis was done from laparoscopic biopsy. Combination of QuantiFERON-TB, ADA and FDG-PET was useful in diagnosing tuberculous peritonitis.