A 62-year-old woman had suffered from interstitial pneumonitis caused by collagen disease and had received steroids and immunosuppressants for twenty years. She was diagnosed as pseudomyxoma peritonei by CT examination and underwent palliative cytoreduction two years ago, but peritoneal relapse occurred one year later. At her first visit to our office, she complained of abdominal distension and respiratory distress of Hugh-Jones classification 2-3. CT showed interstitial pneumonitis and a massive intra-abdominal mucinous tumor. Complete cytoreduction by peritonectomy procedures, combined with intraperitoneal chemotherapy with 50mg of cisplatin, was performed. The duration of the operation was 860 minutes and the blood loss was 7, 000 mL. Postoperative steroidal replacement was performed and neither acute exacerbation of interstitial pneumonitis nor any other severe complication occurred. Today, in the 3-year follow-up period, she is doing well without any sign of recurrence of pseudomyxoma peritonei.