An 83-year-old woman received 8 courses of chemotherapy(mogamulizumab)for adult T cell leukemia in the hematolo- gy department of our hospital, after which she achieved complete remission and was followed up with chemotherapy(VP/ MST: sobuzoxane/etoposide)as an outpatient. Later, diarrheal symptoms appeared, and detailed examinations led to a diagnosis of cancer of the ascending colon. Although no distal metastasis was found, breast cancer was also revealed in the C area of the right breast. The general status of the patient was favorable; thus, right pectoral muscle-conserving mastectomy and concomitant sentinel lymph node biopsy were performed through laparoscope-assisted extended right hemicolectomy. The postoperative course was favorable, and she was discharged on hospital day 7. The excised tumors were pathologically diagnosed as stageⅠ breast cancer and stage Ⅲa colorectal cancer. Chemotherapy(VP/MST)was administered without adjuvant chemotherapy. Presently, 18 months after surgery, complete remission of adult T cell leukemia has been maintained, without metastasis and recurrence of cancer of the ascending colon and breast cancer.