A 74-year-old man was admitted to the hospital because of a sense of abdominal fullness. Plain X-ray film of the abdomen showed lineal gas in the wall of the caecum, ascending colon and transverse colon. Moreover, free gas was found under the right subphrenic space. Under a diagnosis of pneumatosis coli with free gas in the abdomen, high density oxygen treatment was performed by T piece of 70% in concentration, 8l in flow after tracheotomy. The patient was discharged from the hospital after a cure on roentgenologic findings, but experienced a recurrence 2 months after discharge. As his condition was slighter than that at the first admission, high density oxygen treatment was performed by oxygen mask of 100% in concentration, 10l in flow. After discharge from the hospital, for the purpose of prevetion of recurrence, high density oxygen treatment was performed by oxygen mask of 90% in concentration, 3l in flow at home. The patient has now been followed for 1 year without any evidence of recurrence.