Mouth props are widely used tools in clinical practice: from general dental practice to oral care in residential elderly care facilities. Mouth props are particularly useful for keeping the mouth open while administering oral care to persons in residential elderly care facilities who are unable to communicate with others. Currently, oral bites and such devices are used mainly as mouth props in such cases. However, patients who have difficulty communicating resist the insertion of the tools using their lips, which they often clench and defiantly refuse to open their mouths. On such occasions, medical personnel and caregivers must wait until the range of mouth opening is sufficient to insert a tool; this is a heavy burden for caregivers responsible for administering dental and oral care during house calls. To resolve these difficulties, we developed a mouth prop, which is easy to use by both medical personnel and caregivers, can be inserted through a smaller range of mouth opening, can open the mouth by the application of force via the tool after insertion, and can prevent injury inside the oral cavity.