A 55-year-old woman was seen at the hospital because her stools were positive for occult blood at medical checkup. Barium enema study revealed a polyp about 10mm in diameter in the rectosigmoid. Colonoscopy demonstrated a 9× 7mm sessile poly with central depression at the same portion and a polypectomy was carried out. Since histological examination of the polypectomy specimen showed adenocarcinoma with positive polypectomized stump, high anterior resection was performed. Resected specimen revealed moderately differentiated adenocarcinoma which remained in the colonic wall and invaded the subserosal layer with vessel invasion and lymph node metastasis. Eleven cases of small advanced colorectal cancer under 10mm in size have been reported in Japan including our case. We should take a careful attitude to apply the polypectomy for small colorectal lesions.