A lip closing tool can be configured to encourage an infant or the like to close his/her mouth unconsciously and keep his/her mouth closed, thereby encouraging formation of a habit of nasal respiration. The lip closing tool can include a lip contact portion that is contacted by the outer surface of the lips during use, and a main body that is configured to be disposed inside the oral cavity during use. The main body can include a teeth contact portion that is disposed on the outer surface of the teeth during use and extends in a left-right direction. A thin lip contacting portion can connect the lip contact portion to the teeth contact portion and can be contacted by the upper and lower lips during use. The teeth contact portion can be formed entirely and integrally from a flexible material. Stimulation portions can be provided by enlarging the volume of the teeth contact portion near the end of right and left paths thereof.提供一種可促使嬰幼兒等無意識地閉合嘴巴，而維持嘴巴閉合狀態，可促使其養成以鼻呼吸習慣化之嘴唇閉合器。；其具有擋接在嘴唇外面之嘴唇外方配置部12；及，配置在口腔內之本體11；如此的嘴唇閉合器10，前述本體係具有：被配置在齒列外面，在左右方向延伸之齒列外方配置部13；及，連結前述嘴唇外方配置部及前述齒列外方配置部，而藉上下的嘴唇以擋接之較薄的嘴唇擋接部16，且齒列外方配置部其全體係以柔軟的材料一體所形成，其具有前述齒列外方配置部的左右路右端附近被構成具有大體積之刺激部15、15。
