This report is of a case of tuberculous peritonitis that developed during antituberculous chemotherapy. A 54-year-old man had been diagnosed as all-drug susceptible pulmonary and intestinal tuberculosis, and treatment with isoniazid, ethambutol and rifampicin had been initiated. About 5 months later, while still undergoing therapy, a large amount of ascites developed. A diagnostic laparoscopy was performed but due to the adhesion between the greater omentum and the parietal peritoneum, intestinal perforation occurred. An emergency operation was performed and the diagnosis of tuberculous peritonitis was confirmed. There are few reports of abdominal tuberculosis developing during antituberculous chemotherapy. In this case a paradoxical response may have been involved in the pathogenesis.