A female patient (14 years-old) with mental retardation was scheduled for ambulatory general anesthesia to treat peri-apical abscess and multiple dental caries. She had got cleft palate plasty at 5 years, but there was no past history of difficulty airway during general anesthesia or airway obstruction. Following induction of anesthesia using an inhalational anesthetic technique, conventional naso-tracheal intubation was tried. However, with conventional intubation technique we could not insert tube. And following several trial of intubation with laryngoscope, she developed an upper airway obstruction. Her lungs could not be ventilated using a facial mask and oxygen saturation was decreased. A #3 laryngeal mask airway (LMA) was inserted immediately, which allowed us to ventilate her lungs and restore the oxygen saturation. And dental treatment was carried out under LMA insertion successfully.