Invagination in adults is rare compared to children. We experienced 3 sigmoid colon cancers in which symptoms of invagination were discovered. Because the tumors in 2 of the 3 cases could be reduced, operations were electively performed. The tumor in the remaining case could not be reduced and an urgent operation was required. Because all cases responded in the early stage, their postoperative progress was good. Invagination in adults is typically caused by an organic disease, particularly cancer. It is necessary to consider not only reduction of the invagination but also treatment of the primary disease causing invagination. It is important that the malignant symptoms be detected promptly in order to develop a suitable treatment strategy.