An 82-year-old woman underwent surgery for gastric cancer at another hospital in May 2007. The pathological diagnosis was pT4a, pN2, M1, CY1, pStage Ⅳ. Although postoperative chemotherapy was administered, recurrence was observed on the abdominal wall in March 2014, and she was treated usingchemotherapy and resection. Intestinal obstruction due to peritoneal metastasis occurred in December 2017 and mid-July 2018 but symptoms improved with conservative treatment. In late August 2018, she was unable to eat and was readmitted to the hospital. Serum Na level at admission was low at 120 mEq/L, and Na correction was performed. Hyponatremia did not improve, and the serum Na level continued to decrease to 115mEq/L on the 14th day of hospitalization. Plasma osmolality was 229mOsm/kg, urine osmolality was 323mOsm/kg, and urine sodium concentration was 56mEq/L. Diagnosis of SIADH was made according to diagnosis standards. Hyponatremia improved by fluid restriction and Na correction. Subsequently, her peritoneal metastasis exacerbated, and she died in mid- October. We would like to report a case of SIADH in an elderly patient with advanced gastric cancer.