A mouthpiece comprising an ethylene-vinyl acetate copolymer having a vinyl acetate content of from 12 to 33 wt %, and the process for producing the same comprising the steps of:preparing a thermoplastic elastic sheet material formed to be increased in thickness from one end to the other end;preparing a teeth impression cast of a user who is going to use a mouthpiece to be produced; andpressing said teeth impression cast against said thermoplastic elastic sheet material in a softened state so as to transfer a shape of said teeth of said user onto said thermoplastic elastic sheet material to thereby obtain a mouthpiece which is set to be thicker at a portion corresponding to a front teeth position than at a portion corresponding to a molar teeth position.