A 57-year-old man with advanced gastric cancer and multiple liver metastases was referred to our hospital. He underwent a palliative gastrectomy to treat hemorrhage, and S-1 and cisplatin therapy was administered. After 7 courses of chemotherapy, a new liver metastatic lesion and a tumor thrombus in the right portal vein appeared. Moreover, the serum level of ammonia was elevated(296 mg/dL)following a consciousness disorder. Enhanced CT revealed an inferior mesenteric vein to left renal vein shunt, which led to the diagnosis of portal systemic encephalopathy due to portosystemic shunt. Percutaneous transvenous coil embolization was performed. The serum ammonia level decreased, and the encephalopathy disappeared. As a result, he was able to continue chemotherapy.