Diffusely infiltrating primary colorectal carcinoma is a rare disease. This type of carcinoma extensively spreads beneath the mucosal layer. Clinically, it resembles inflammatory colorectal disease and metastatic carcinoma and is difficult to diagnosis. Moreover, the prognosis is extremely poor. A 66-year-old man was admitted to our hospital because of lower abdominal pain. Barium enema showed circumferential long segment narrowing in the proximal sigmoid colon without definite mass or ulceration and abdominal CT demonstrated colonic wall thickening. So it was diagnosed as inflammatory condition by image study. Colonoscopic examination showed hypertrophied mucosa and luminal narrowing on the 40 cm from the anal verge, but the oral side couldn't be examined because of stenosis. Biopsy revealed poorly differentiated adenocarcinoma, and left hemicolectomy was done. The pathology showed diffusely infiltrating adenocarcinoma with metastasis in regional lymph node and mesentry involvement.