To provide a mouth guard-preparing sheet from which a mouth guard to be used for preventing teeth and their surrounding tissues from a trauma occurred mainly in sports and having a uniform thickness between the right and left sides thereof can be prepared, without causing a problem in the thickness of an anterior teeth portion becoming thin in the preparation of a custom-made mouth guard by heat-sucking. The mouth guard-preparing sheet is made of a thermoplastic resin having a plane shape substantially coincident with a shape of a fixed frame of a heat-sucking type mouth preparation device and provided with a mouth guard portion from a site to be contacted with a labial surface side of a gingiva of an anterior teeth portion to a molar portion side and an elongation tab portion in an exterior direction of a mouth guard portion, wherein said mouth guard portion has a thickness of 1.510 mm; said elongation tab portion has a thickness of 0.10.95 times the thickness of said mouth guard portion; and a stepped portion between the mouth guard portion and the elongation tap portion is in a straight line.