When obstructive colitis becomes fulminant, it is known as 'acute necrotizing colitis'. We report a rare case of acute necrotizing colitis due to sigmoid colon cancer, in which shock status occurred within ten hours of onset. A 79-year-old female with acute abdominal pain was transported to our hospital with acute shock. Abdominal enhanced computed tomography revealed thickening of the wall of the sigmoid colon and marked dilation of the proximal colon. Emergency surgery was performed with the intraoperative findings of severe sigmoid colon stenosis and proximal dilation of the colon without perforation, and a large volume of putrid ascitic fluid. The intestine was proximally dilated and black in color, from the sigmoid colon to the ileum 60 cm proximal to the terminal ileum, suggesting acute necrosis. Total colectomy with 80 cm resection of terminal ileum and ileostomy was performed. Whereas acute necrotizing colitis is a rare condition and its etiology remains obscure, the chronic ischemic state must play some role. Our patient was of advanced age and had diabetes mellitus and hypertension. These factors might lead to a chronic ischemic state of the bowel due to arteriosclerosis. In addition to the underlying condition, massive bacterial reflux into the ileum from the colon might cause the capillary vasoconstriction of the bowel that led to her critical state.