The mouthpiece has an upper jaw piece to be attached to the upper dentition, a lower jaw piece to be attached to the lower dentition, a connecting member for positioning the lower jaw piece with respect to the upper jaw piece, and a recess. A locking portion formed on the inner wall surface of the upper jaw piece, a convex portion to be mounted in the concave portion of the locking portion, and a sandwiching portion sandwiched between the upper jaw piece and the lower jaw piece A spacer.マウスピースは、上顎の歯列に装着される上顎用ピースと、下顎の歯列に装着される下顎用ピースと、上顎用ピースに対して下顎用ピースを位置決めする連結部材と、凹部を有し、上顎用ピースの内壁面に形成された係止部と、係止部の凹部に装着される凸部と、上顎用ピースと下顎用ピースとの間に挟持される狭持部と、を有するスペーサと、を備える。
