Nivolumab has promising efficacy for treating various advanced malignant tumors, although it has been reported to induce a wide range of autoimmune adverse effects. We herein report the case of a patient with metastatic lung adenocarcinoma who developed adrenal insufficiency after 12 cycles of nivolumab treatment. Endocrine test results supported a diagnosis of isolated adrenocorticotropic hormone deficiency due to hypophysitis, and replacement therapy using hydrocortisone has been successful. Although hypophysitis is a rare immune-related adverse event that is associated with nivolumab therapy, clinical awareness is essential, as this condition can be life-threatening and requires prompt treatment.