We report a case of refractory SIADH associated with carboplatin administration. The patient was a 74-year-old woman with a local recurrence of nasopharyngeal carcinoma. Pembrolizumab was administered to treat the local recurrence, but the disease progressed. Carboplatin, 5-FU, and cetuximab were administered as secondary treatment. The patient presented with headache and anorexia on the 7th day of administration, and lethargy, upper limb convulsions and severe hyponatremia （110mEq/L） on the 8th day. 3％ hypertonic saline was administered, and consciousness improved 6 hours after administration. The results of the examination on the 8th day met the diagnostic criteria for the syndrome of inappropriate antidiuretic hormone secretion （SIADH）, which required differentiation from renal salt wasting syndrome （RSWS）. There was no obvious dehydration, so the patient was diagnosed with SIADH. From the 11th day, serum sodium decreased again. Although fluid restriction was implemented from the 18th day, serum sodium sometimes fell below the reference value, so a vasopressin V₂ receptor antagonist was administered from the 23rd day. Serum sodium improved to within the normal range on the 29th day, and the patient was discharged on the 30th day.