Caudal block is the world-widely used technique for an effective postoperative analgesia in children undergoing OPD-based infra-umbilical surgery. Because the caudal block is usually performed with a single-shot, proper dosage is required to achieve sufficient analgesic levels. Several clinical researches have been performed to assess the drug spread levels using mathematical equation, X-ray evaluation, and ultrasonography during caudal block in children. In our previous study using fluoroscopy, we obtained the results that the established weight-based doses provided adequate block levels for each type of surgery.