An 80-year-old woman was admitted to our hospital with anal bleeding. After close examinations, she was diagnosed with anal canal cancer [cT4(suspected vaginal invasion) N0M0 cStage III A] and gastric cancer (cT1N0M0 cStage IA). Radiation therapy to the pelvic region and chemotherapy with oral administration of S-1 were performed, and then laparoscopic assisted abdominoperineal resection of rectum·D2 dissection and distal gastrectomy·D2 dissection·B-I reconstruction were carried out. The postoperative pathology resulted in pCR in the anal canal cancer and ypStage IA in the gastric cancer. After the surgery, her clinical course has been followed without adjuvant treatment.Since a nodule was recognized in the region B of the left mammary gland, we conducted a histological examination. The histological diagnosis was primary breast cancer (solid tubular carcinoma), and we performed left total mastectomy and axillary lymph node dissection (Kodama). Pathology could not rule out a possibility of metastasis of gastric cancer. HNF4a was positive by immunostaining, and mammary gland metastasis of gastric cancer was diagnosed. There have been no reports of cases using HNF4a for diagnosis of such mammary gland metastasis of gastric cancer so far. The use of HNF4a for diagnostic purpose seems very rare.