Metastasis of breast carcinoma to the stomach is relatively uncommon, although metastasis to other organs such as lung, bone, and lymph nodes is not rare. It may cause difficulty in differentiating from primary gastric carcinoma. We report a case of signet ring cell carcinoma of the breast with metastasis to the stomach with illustrations of histologic findings of both lesions. The results of immunohistochemical stainining with GCDFP-15 (gross cystic disease fluid protein-15), that can be used to differentiate primary gastric signet ring cell carcinoma and metastatic mammary signet ring cell carcinoma, are described.