A 51-year-old female had been diagnosed with a hemangioma in the hepatic segment 6 (S6). After a 6-year follow-up, enlargement of the tumor was detected. The tumor was clearly enhanced in the arterial phase, and the enhancement remained in the portal phase on computed tomography (CT). Although the primary differential diagnosis on CT was hepatocellular carcinoma (HCC), we worried about the possibility of other vessel system tumors because the tumor remained to be enhanced at the portal phase for HCC and all tumor markers of HCC were negative. We performed angiography to determine the tumor nature and to seek other tumors. Angiography showed tumor stain at the hepatic S6 with an early obvious drainage vein from the tumor flowing through the right hepatic vein into the inferior vena cava. In addition to tumor stain and the drainage vein, there were many small poolings of contrast medium in the whole liver, which were suspected as dilatation of the hepatic peripheral artery. We suspected the tumor as a benign tumor such as hepatocellular adenoma or focal nodular hyperplasia, but the possibility of HCC could not be ruled out. Hepatic posterior sectionectomy was done to completely remove the drainage vein with the tumor. Intraoperative histological examination revealed the tumor as not malignant and not HCC. Later, immunohistochemical analysis uncovered that the tumor had high expression of HMB-45 and, therefore, the final diagnosis was angiomyolipoma. We think that detecting an early drainage vein from the tumor would be a key point for diagnosing hepatic angiomyolipoma.