To prevent a snore by prohibiting the backward displacement of the mandible.

This device has a main body 5 to be freely attachably and detachably fixed to the upper teeth 3 via fixing means 2. The rear end of the main body 5 is projectingly provided with a detaining wall part 7 which can be pressed to the rear surface part 15 of the molar 14 existing in the last part of the mandible. This engaging wall part 7 engages with the rear surface part 15 of the molar 14, by which the backward displacement of the mandible is prohibited and the snore is prevented even if the mouth is opened during sleeping.