We report a case of ruptured stercoral ulceration due to chronic constipation which is caused by rectal carcinoma. This case suffered from difficulty of stool passage for 5 months. Periumbilical pain and current-jelly stool were experienced before his admission. Physical examination revealed diffuse abdominal rebounding pain and laboratory data showed leukocytosis. Computed tomography demonstrated marked dilatation of the sigmoid colon with stool impaction due to neoplastic growth in the rectosigmoid junction. Thickening and edematous change of the colonic wall were noted. There was amorphous material with gas in the mesocolon, which indicated fecal peritonitis. Emergent operation with Hartman's procedure and left colostomy was performed. Diffuse pressure gangrene of the sigmoid colon wall with a perforating hole was identified. Pathologically, the resected colon specimen showed non-specific acute and chronic inflammatory change. The perforating hole was surrounded by a necrotic border of ulcerative mucosa. After the operation, pelvic drainage was undertaken for 1 month and then the patient was discharged uneventfully.