Primary adrenal insufficiency due to tumor involvement is very rare. We herein report a case of adrenal insufficiency caused by cancer metastasis to a remaining single adrenal gland in a renal cell carcinoma patient who had undergone radical nephrectomy including removal of the right adrenal gland. The patient presented with abdominal discomfort, nausea, and vomiting which had begun about 3 months earlier. A rapid adrenocorticotropic hormone (ACTH) test showed no cortisol response with upper normal level of baseline ACTH. The patient was treated with prednisolone after diagnosis of primary adrenal insufficiency due to adrenal metastasis. The symptoms improved after prednisolone replacement. The case reminds clinicians the diagnosis of primary adrenal insufficiency due to tumor metastasis should be considered in patients with nonspecific gastrointestinal symptoms.