A case of renal angiomyolipoma rapidly growing during pregnancy with tumor thrombus into the inferior vena cava is reported. Angiomyolipoma in a 31-year-old woman was incidentally found by ultrasonography. CT scan revealed a fat-containing tumor in the right kidney with 4 cm in diameter. The patient was followed at outpatient clinic without any treatment. Fifteen months later, the post-delivery follow-up CT scan showed that tumor size had grown up to 11 cm in diameter. Although laparoscopic right nephrectomy was tried, open transperitoneal right nephrectomy was performed because the tumor thrombus extending into the inferior vena cava was found during the laparoscopic procedure. Pathological diagnosis was angiomyolipoma of the kidney. There are several reports that indicate angiomyolipoma grows rapidly during pregnancy. Our case demonstrates that careful follow-up is required for angiomyolipoma in women with possible conception and delivery.