A 40-year-old man was admitted due to thirst, general malaise, and swollen cervical lymph nodes. Cervical lymph node biopsy revealed moderately differentiated adenocarcinoma, and he was diagnosed lung cancer with hypothalamic metastasis. We gave him chemotherapy and hypothalamic irradiation. He had mild hydrodipsia at the first examination, which progressively worsened. He was given a diagnosis of diabetes insipidus caused by hypothalamic metastasis. We gave him desmopressin acetate, and his symptom improved. Predonisolone was prescribed, because of low levels of adrenocorticotropic hormone in the blood. After chemotherapy and irradiation of hypothalamus, metastatic focus diminished and desmopressin acetate was tapered. Hypopituitarism caused by hypothalamic metastasis is rare.