PROBLEM TO BE SOLVED: To prevent oral breathing effectively by having a simple structure, being positioned at the center of upper and lower teeth in conjunction with opening of the mouth, and being prevented from moving in the oral cavity during sleep. A mouth breathing prevention device and a mouth breathing prevention sheet that can be used are provided. An oral breathing preventer includes a plate-shaped base portion (1) which is curved so as to be aligned with teeth, and a plate-shaped meshing portion which is formed on a part of a curved inner surface of the base portion (1) and is bitten by teeth. 2, a projection 3 formed at the tip of the meshing portion 2 in parallel with the base portion 1, and a meshing portion 4 formed at both tip ends of the projection 3 and bent toward the base portion 1. [Selection diagram]【課題】簡素な構造を有し、口の開きに連動して上歯と下歯の中心に位置し、睡眠中に口腔内で移動してしまうことが防止され、口呼吸を効果的に防止できる口呼吸防止具及び口呼吸防止シートを提供する。【解決手段】口呼吸防止具は、歯の並びに沿うように湾曲した板状の基部１と、この基部１の湾曲内側の面の一部に形成され、歯により噛み込まれる板状の噛合部２と、この噛合部２の先端に基部１に平行に形成された突起３と、突起３の両８先端に形成され、基部１に向けて屈曲した噛込部４とから構成されている。【選択図】 図１
