The mouthpiece according to a first aspect of the present invention comprises a pair of opposing upper jaw mouthpiece and lower jaw mouthpiece, a pair of positioning members that is provided in either one of the upper jaw mouthpiece and the lower jaw mouthpiece and to be able to carry out positional adjustment in a front-back direction, and a pair of stoppers that is provided in the other one of the upper jaw mouthpiece and the lower jaw mouthpiece, regulates a rearward displacement of the lower jaw mouthpiece by coming into abutment with the positioning members, and regulates lateral displacements of the upper jaw mouthpiece and the lower jaw mouthpiece by coming into abutment with the either one of the upper jaw mouthpiece and the lower jaw mouthpiece.
