A 78-year-old man with advanced gastric cancer was treated with S-1 and oxaliplatin chemotherapy. He developed hiccups and nausea, and was diagnosed with hyponatremia (serum Na: 120 mEq/L) on day 6 of the first treatment course. Because of his increased urinary Na excretion and relatively high ADH values, he was subsequently diagnosed with chemotherapy-induced syndrome of inappropriate secretion of antidiuretic hormone. The patient recovered after an infusion of hypertonic saline. Although S-1 was restarted, hyponatremia did not recur. We suspected adverse drug reactions to ACE inhibitors and K-sparing diuretics in our case of hyponatremia.