A 71-year-old woman with peri-anal pain and anal bleeding was found to have a tumor in lower rectum in colonoscopic examination, which was histologically diagnosed as poorly differentiated adenocarcinoma. Computed tomography and magnetic resonance imaging showed a tumor 5 cm in diameter deeply infiltrating mesorectum, and swelling of intra-pelvic lymph nodes. At the first operation, infiltration of the tumor was so deep that a complete surgical resection was considered to be difficult, and only a sigmoid colostomy construction was done. After chemo-radiation therapy of 50.4 Gy and 5-FU plus levofolinate calcium (Isovorin) infusion, the tumor was reduced in size and lymph nodes were extremely decreased. The patient underwent an abdominoperinial resection 6 weeks after the end of chemo-radiation, and pathological examination revealed a complete replacement of tumor with necrosed tissue, and no viable tumor cell was found. Poorly differentiated adenocarcinoma of the colon is reported to be highly malignant and associated with poor prognosis. Preoperative chemo-radiation therapy can be a promising candidate for adjuvant treatment of locally-advanced poorly differentiated adenocarcinoma of the rectum.