A 72-year-old woman with slowly progressive type 1 diabetes (SPIDDM) was admitted to our hospital because of increasing abdominal pain and diarrhea. The patient was diagnosed with nonocclusive mesenteric ischemia (NOMI), and a subtotal colonectomy was performed successfully. The resected sample revealed transmural gangrenous necrosis of the colon and rectum. This case is interesting because the severe NOMI occurred in a SPIDDM patient without common predisposing events such as hypoperfusion. Prolonged generation of reactive oxygen species in SPIDDM, together with the decline in adaptive response to oxidative stress with aging, might be an exacerbating factor for ischemic injury in the elderly.