A 52-year-old woman presented with an 8-month history of epigastric pain, nausea, and weight loss. One year before, she was diagnosed with breast cancer. During the postoperative chemotherapy, she developed epigastric pain and nausea. As a result, she gradually lost 12 kg of her body weight. We performed upper gastrointestinal endoscopy, which revealed mild erosive gastritis. After the treatment with a proton pump inhibitor, her symptoms persisted. Before the admission, mild hypercalcemia was pointed out. Fluid replacement didn't improve hypercalcemia. We assessed adrenocortical function, which showed that her serum cortisol and adrenocorticotropic hormone were decreased. Through loading tests, we established diagnosis of isolated adrenocorticotropic hormone deficiency. She was treated with hydrocortisone. Soon after the treatment, her serum calcium level returned to normal and her symptoms improved. In a case of hypercalcemia unresponsive to fluid replacement, we recommend ruling out adrenal insufficiency after excluding more common diseases which induce hypercalcemia.