We experienced a case of pneumatosis cystoides intestinalis with intraperitoneal free air which is the 13th case report in Japan. A 62-year-old man, who had been in palliative care for terminal cancer of the left lung and taken morphine sulfate for the pain control, was brought into the hospital because of severe abdominal pain. Physical examination showed left lateral abdominal pain. Abdominal X-ray film revealed free air. Abdominal CT scan showed pneumatosis in the intestinal wall and mesenterium. A possible occurrence of intestinal perforation could not be denied and laparotomy was performed. A number of small cysts were present in the intestinal wall and mesenterium, however, no perforation of the intestine was identified. The definite diagnosis of pneumatosis cystoides intestinalis was made. After irrigation a drain was placed and the operation was over. The postoperative course was uneventful and the patient was discharged from the hospital on the 16^th postoperative day. It is inferred that the cause of pneumatosis cystoides intestinalis in this case is chronic constipation induced by taking morphine sulfate and an α-glucosidase inhibitor. When free air is found, it is important to keep the possibility of perforation of intestines in mind. If it is difficult to make the difinite diagnosis, exploratory laparotomy should be considered for suspected perforation.