Spontaneous retroperitoneal hemorrhage is a rare condition with a poor prognosis. It is usually treated conservatively unless vital signs are unstable. We report a case of a 60-year-old male patient with acquired cystic kidney disease who presented to ED due to continuous left lower abdominal pain. His vital signs were normal, and a physical examination revealed tenderness in the lower left abdomen. CT revealed a hematoma with extravasation around the left kidney. He was treated conservatively due to stable vital signs and laboratory data. However, a decrease in the hemoglobin level and an increase in hematoma were observed, and blood transfusion and transcatheter arterial embolization were performed on the second day of admission. In such cases, early embolization should be considered even when the patient's general condition is stable.