A 25-year-old woman, at 38 weeks of gestation, suddenly developed a left lumbar pain. Emergency cesarean section was performed to rescue the fetus since there was fetal bradycardia. At operation, a slight amount of retroperitoneal hematoma was observed, but was left without further examination. After the surgery, the patient complained of severe lumbar pain and fell into shock. A ruptured aneurysm of the left renal artery was suspected by computed abdominal tomography. At the second operation, a massive hematoma was observed in the retroperitoneal space, and a ruptured aneurysm of the left renal artery was confirmed after mobilization of the left kidney with the aid of manual hemostasis. Aneurysmectomy with arterial reconstruction was not considered feasible, and complete left nephrectomy was done because the patient was severely ill, in addition to the fact that the aneurysm was located at the trifurcation of the left main renal artery. The postoperative course was uneventful. On the surgical specimen the aneurysm measured 27mm in circular length, the lower half of which was ruptured. Histologically, the internal elastic lamina was torn, and the medial smooth muscle fibers were sporadically replaced with fibroblastic cells. The prognosis of ruptured aneurysm of a renal artery is often lethal. Therefore it is very important to suspect this lesion when we have to manage physical problems with patients in pregnancy so that we can make an early diagnosis with better results.