We report a case of pneumoperitoneum at the end stage of lung cancer. The patient was an 81-year-old man who experienced only mild abdominal pain and fullness. The pneumoperitoneum spontaneously disappeared. Ruptured viscera and peritonitis were not observed at autopsy. We speculated that air had moved from the lung cancer lesion to the mediastinum, and then through the retroperitoneal space into the peritoneal space. When pneumoperitoneum is observed, lung cancer and other intrathoracic diseases should be suspected as causes. Also, it may be necessary to carefully examine the patient for fever, general appearance, abdominal findings, and laboratory findings, including white blood cell count and C-reactive protein level, and on that basis differentiate from ruptured viscus, which requires an emergency laparotomy.