Anesthesia will be induced by propofol 1mg/kg via intravenous line. Once loss of eyelash reflex occurred, ultrasonography of antral area will be started by same physician. After administration of muscle relaxant (rocuronium 0.2-0.4mg/kg), manual ventilation will be performed with respiratory rate 20 breathes/min, I:E ratio of 1:1 and tidal volume with 9-10mL/kg. The pop-off valve will be set at 15cmH2O at fixed gas flow of 500mL/min of oxygen. Pressure-controlled ventilation will be performed with respiratory rate 20 breaths/min, I:E ratio of 1:1 and peak airway pressure will be set to get a tidal volume of 9-10mL/kg. The peak airway pressure during facemask ventilation will be checked. Another physician will auscultate the epigastric area to detect gastric insufflation during facemask ventilation. Ultrasonography of antral area will be checked once again after 3 minutes of facemask ventilation.

The primary outcome is the difference of peak airway pressure produced by manual ventilation group and pressure-controlled ventilation group during induction of anesthesia.

All patients' antral area of stomach will be calculated by ultrasound after 3 minutes-facemask ventilation to estimate the quantity of gas insufflation.