A 77-year-old male was admitted to our hospital for a bulky abdominal mass. He had a history of appendectomy under the diagnosis of appendiceal rupture 23 years previously. He also had received a radical lung resection for an early lung cancer 2 years earlier in another hospital. Tentative diagnosis of peritoneal metastases from the lung cancer was made. He then received 3 courses of chemotherapy, but failed to reach a remission. The final diagnosis of pseudomyxoma peritonei was made by means of abdominocentesis, and he underwent debulking surgery. However, he died on day 56 after the surgery. Pseudomyxoma peritonei requires careful observation, as it has the possibility to be detected after a long-term follow-up period of more than 20 years.