PROBLEM TO BE SOLVED: To enable production of a mouthpiece regardless of difference of oral cavity size (such as dental alignment) among users while preventing a gap from being formed near the base of an inner side of a tooth (i.e., the vicinity of a boundary between the tooth and the gum).SOLUTION: A mouthpiece production method includes the steps of: making a user bite a tray having a first material 41 is disposed on a first surface 311 capable of facing a palate or a tongue of the user, thereby pushing the first material against the palate or the tongue to make the first material into a wall material capable of facing a palate side surface 51a or a tongue side surface of the dentition； and making the user bite the tray having a second material 42 disposed on a second surface 312 capable of facing an occlusal surface of the dentition of the user, thereby applying pressure from the dentition 51 to the second material surrounded by the second surface and the wall material to form a female mold of the dentition in the second material.SELECTED DRAWING: Figure 6【課題】ユーザ毎の口腔のサイズ（歯の配列態様等）の違いによらずに、歯の内側側面の付け根付近（つまり、歯と歯肉との境界付近）に空隙が形成されることを防止しながらマウスピースを製造する。【解決手段】ユーザの口蓋又は舌に対向可能な第１表面３１１に第１材料４１を配置したトレーをユーザに噛ませることで第１材料を口蓋又は舌に押し付け、第１材料を歯列の口蓋側側面５１ａ又は舌側側面に対向可能な壁材にし、ユーザの歯列の咬合面に対向可能な第２表面３１２に第２材料４２を配置したトレーをユーザに噛ませることで第２表面と壁材とに囲まれた第２材料に歯列５１から圧力を加え、第２材料に歯列の雌型を形成する。【選択図】図６
