We report a case of a seventy-year-old woman with syndrome of inappropriate secretion of antidiuretic hormone (SIADH) and adrenal insufficiency induced by Rathke's cleft cyst. She experienced nausea, vomiting, diarrhea, and headache and disturbance of consciousness induced by hyponatremia at a serum sodium level of 100mEq/l. In spite of severe hyponatremia, urinary sodium excretion was not suppressed and serum osmolality (270mOsm/kg) was lower than urine osmolality (304mOsm/kg), and arginine vasopressin (AVP) remained within normal range. SIADH was diagnosed because she was free from other diseases known to cause hyponatremia such as dehydration, cardiac dysfunction, liver dysfunction, renal dysfunction, hypothyroidism, and adrenal insufficiency. Cranial computed tomographic (CT) scan and cranial magnetic resonance (MR) imaging showed a cystic lesion of approximately 2cm in diameter in the pituitary gland. These images suggested that the cystic lesion was a Rathke's cleft cyst, which was the cause of SIADH. Water restriction therapy normalized her serum sodium concentration and improved her symptoms. After one year, she suffered from general fatigue, appetite loss, fever, and body weight loss (5kg/2 months). She had neither hypotension nor hypoglycemia, but her serum sodium level was low and serum cortisol, ACTH, and urine free cortisol were very low. Therefore, secondary adrenal insufficiency was suspected and diagnosed by stimulation tests. After start of hydrocortisone replacement therapy (10mg/day), her symptoms disappeared. In conclusion, Rathke's cleft cyst should be kept in mind as a potential cause in a patient with SIADH, hypopituitarism, and/or adrenal insufficiency.