A female patient in her 50s who underwent chemotherapy for left primary breast cancer presented with cancerous pleurisy and disseminated intravascular coagulation. Esophagogastroduodenoscopy and liver biopsy revealed gastric and liver cancer. Distinguishing between primary and metastatic cancer by pathological findings is difficult using hematoxylin and eosin staining. We diagnosed and treated simultaneous primary breast cancer (ER-positive) and gastric cancer with liver metastasis (ER-negative), based on differences in estrogen receptor expression. The patient lived for 10 months with chemotherapy. After death, an autopsy was performed because the endoscopic results were atypical for primary gastric cancer, and additional immunohistochemical studies indicated gastric metastasis of breast cancer.