We report herein the case of a ruptured liver abscess that resulted in pneumoperitoneum. A patient with diabetes mellitus presented with symptoms of acute abdomen. The plain abdominal radiograph and computed tomography findings revealed abdominal free air and a gas-containing liver abscess, whereby a diagnosis of a ruptured liver abscess was made. An emergency operation was performed, and the abscess was drained followed by peritoneal lavage and the administration of appropriate antibiotics. To the best of our knowledge, very few cases of spontaneous pneumoperitoneum occurring secondary to the rupture of a gas-containing liver abscess have been encountered in Japan.