A 90-year-old woman was referred to our hospital because of abdominal distension and pretibial edema. No tumors were palpable in either breast. Her abdomen was distended and palpitation was noticed. Laboratory examination revealed hypercalcemia and elevated levels of tumor markers(PIVKA- II , CEA, CA19-9, SCC). Contrast-enhanced computed tomography(CT) showed pleural effusion and ascites, an approximately 17-mm high-contrast mass in the D area of the left breast, and osteolysis of the dorsal lumber spine. We initiated treatment for pleural effusion, ascites, and hypercalcemia. However, on her 11th day in the hospital, hematemesis was observed and the patient died. We suspected metastasis of a malignant tumor, but a primary lesion had not been identified; therefore, an autopsy was performed. Autopsy showed a 17-mm mass in the D area of the left breast. Histopathological examination showed the growth of a signet-ring cell tumor that was immunohistochemically ER-positive, PgR-positive, and E-cadherin-negative. The patient was diagnosed with primary signet-ring cell carcinoma of the breast derived from lobular carcinoma. We reported a rare case of primary signet-ring cell carcinoma of the breast with a review of literature.