The aim of this study is to evaluate the drug spread level with a weight-based doses using ultrasonography during caudal block in children and assess the reliability of the real time ultrasonography to determine the drug spread levels. After general anesthesia, the patient will be placed in lateral decubitus position for caudal block. Total 1.5 ml/kg of 0.15% ropivacaine will be injected into the caudal space. The spread level will be step-wise evaluated using ultrasonography at the injected drug dose of 0.5 ml/kg, 1.0 ml/kg, 1.25 ml/kg, and 1.5 ml/kg.