A case of gastric carcinosarcoma with a rare histology is reported. Grossly, a huge ulcerative tumor (Borrmann type 3 tumor, measuring 10 x 6 cm) was observed on the lesser curvature side of the stomach. Microscopically, the tumor consisted of carcinoma and sarcoma components. The main tumor component consisted of an endocrine cell carcinoma exhibiting positive reactions for cytokeratins, chromogranin A, synaptophysin, and CD56. The sarcoma component occupied less than 10% of the entire tumor area and consisted of spindle cells, some of which showed a rhabdomyosarcoma differentiation. Some of the spindle cells also showed an atypical cartilage or osseous differentiation. This sarcoma component exhibited positive staining not only for vimentin and desmin, but also for cytokeratins. Transitions between these two components were occasionally observed. These findings suggest that the sarcoma component in our case most likely originates from the endocrine cell carcinoma component.