A 75-year-old man undergoing operations for rectal cancer was given adjuvant chemotherapy for 6 months. Fourteen months after surgery, peritoneal dissemination was found, so mFOLFOX6 therapy was started. After 34 courses of mFOLFOX6 therapy, peritoneal dissemination progressed. As a second-line treatment, we administered FOLFIRI therapy. The patient visited our emergency unit with the chief complaint of a fever thirteen days after the second course was completed. A chest imaging study showed diffuse infiltrative shadow and ground glass shadow bilaterally. Chemical interstitial pneumonia was suspected from his clinical findings. In the ICU, respiratory care by BiPAP and steroid pulse therapy(mPSL 1 g/day for three days)were performed. Steroid therapy improved his respiratory status and chest image findings, and he was able to leave the ICU on the seventh day. We continued steroid internal use treatment and he was discharged on the 70th day. The agent causing interstitial pneumonia was thought to be CPT-11 from the administration history. The incidence of interstitial pneumonia as an adverse event of CPT-11 is low, but it can be fatal without adequate attention. Steroid pulse therapy was validated for treatment, and it served to prolong life.