A patient with obstructive sleep apnea has been treated by means of a mandibular repositioning appliance made of silicone rubber. The patient is a male and 54 years old with a slim body and complained a excessive daytime sleepiness and unsatisfied sleep. A lateral head plate revealed retruded mandible and narrow A-P diameter in the lower part of oropharynx. Moderate frequency of apnea was found in the initial all-night polysomnographic recording. The mandible has been brought forward by 5 mm and downward by 11 mm, which enlarges the diameter of oropharynx anterio-posteriorly by 2-3 mm. Since the appliance has been inserted during bed-time, the daytime sleepiness and unsatisfied sleep has been eliminated. The second polysomnographic recording revealed significant increment of deep NREM sleep and REM sleep and decrement of arousal during sleep after insertion of the appliance. It is indicated, therefore, that the application of the mandibular repositioning appliance is one of the effective methods for the treatment of obstructive sleep apnea.