 Propofol frequently used as an inducting medication for insertion Laryngeal mask airwayLMA. But propofol alone can be not sufficient for inhibiting upper airway reflex and also result in cardiovascular suppression under enough amount. Dexmedetomidine is a selective alplh2 receptor agonist with analgesic and sedative effects but sparing respiratory function. Therefore propofol combined with dexmedetomidine induction could be more effective in preserve respiration under same condition for LMA insertion than propofol alone. In this study, the investigators investigate the clinical dose of propofol for LMA insertion with premedication of dexmedetomidine, and evaluate the effect of dexmedetomidine. 