A 76-year-old woman presented to our hospital with a right upper abdominal mass detected by an abdominal echography. An abdominal computed tomography showed a 14-cm mass in the right upper quadrant of abdomen, which had remarkably pressed the right kidney and the liver. The right adrenal gland was recognized on the periphery of the mass. The inside was mottled low density, and contrast effect was observed on the margin. T2-weighted magnetic resonance imaging showed a mixture of hyperintense and hypointense regions within the mass. The margin of the mass showed low intensity. She was suspected of retroperitoneal chronic expanding hematoma (CEH) of the adrenal gland. Five months later, surgery was performed because an expansion of the mass was observed on CT. The adhesions were possible to be dissected all around, and we excised the mass without damaging its capsule. She was diagnosed with CEH 20cm in size originating from the adrenal gland based on her medical history and pathological findings. CEH is defined as a hematoma which develops following surgery or injury and expands chronically, and rarely originates from the adrenal gland. Although it was a very huge mass in our case, complete resection including its capsule was possible.