Diverticular disease of the colon in Korea is different from developed Western countries. Diverticulosis leads to the complications in about 20% of cases. The diverticulitis of the right colon is difficult to be differentiated from acute appendicitis. According as increase of elderly populations, incidence of the diverticulitis of the left colon increase. Therefore we should be differenciated from other diseases. In very rare case, frequent recurrence and recovery of the diverticulitis leads to a chronic state with complications such perforation, microabscess, fistula, adhesion and fibrous hypertrophy. These findings may be misdiagnosed as malignancy because of protruding mass and stricture on colonoscopy. We present a case of an pseudotumor that looks like a malignancy in the sigmoid on colonoscopy. After surgical resection, the lesion turned out to be a complicated diverticulitis.