To provide a method of manufacturing mouthpiece by which a mouthpiece of a high accuracy, wherein the tooth shape of a user is reproduced, can easily be manufactured in a short period of time.

A softened thermoplastic sheet material is placed on a dental impression material which is mounted in an impression tray, and the mouthpiece is manufactured by pressing the dental impression material to a tooth from the thermoplastic sheet material side by this manufacturing method. Alternatively, the softened thermoplastic sheet material is pressed to the tooth, and the impression tray in which the dental impression material is mounted is pressed from the top of the thermoplastic sheet material to manufacture the mouthpiece by this method of manufacturing mouthpiece.