A rare case of double cancer with situs ambiguus with polysplenia is presented. A 58-year-old patient was initially diagnosed with an early gastric cancer. On evaluation, the computed tomography of the abdomen demonstrated situs ambiguus with polysplenia. We performed a subtotal gastrectomy with the stomach being reconstructed in a Billroth-II fashion. Three months after the operation, he again visited our department complaining nausea and dysphagia. Examinations confirmed the other oesophageal malignancy with advanced stage. Because of unfamiliarity to situs anomaly and rarity of double cancer, we missed anther coexistent cancer. This is the first case presentation of a double carcinoma occurring in a patient with situs ambiguus with polysplenia. The literature is reviewed and the importance of preoperative evaluation is discussed.