After genernal anesthesia using sevoflurane or desflurane, the investigators can observe many cases of agiation (over 80%) and despite of no report of long-term complication, agitation is a big issue in PACU for doctors and nurses. The investigators use dexmedetomidine for prevention of agiation of preschool aged children. The investigators expect single injection of dexmedetomidine lead to decrease postoperative agitation and pain without any complication like oculocardiac reflex requiring atopine for treatment.