Carcinosarcoma is a rare malignant, biphasic tumor comprised of carcinoma and sarcoma components. In the gastrointestinal tract, carcinosarcoma is most frequently seen in the esophagus and rarely in the stomach. We report a 51-year-old female patient with 2-month-history of epigastric pain and dyspepsia. Endoscopic finding revealed a huge ulcerative lesion that infiltrated from the antrum to the mid-body. An endoscopically taken biopsy revealed poorly differentiated malignant round cell neoplasm. After the palliative subtotal gastrectomy, immunohistochemical studies showed two positive reactions for the epithelial marker and mesenchymal marker. Based on the above findings, the patient was diagnosed with gastric carcinosarcoma. The immunohistochemical analysis is a critical method in making an accurate diagnosis of carcinosarcoma.