A mouthpiece (15) of a vice used to fix a work to the table or a pallet of a machine tool, comprising a base block (42) and a mouthpiece block (43) and a multi-stage tilted sliding surface (50) provided between the front surface of the base block (42) and the rear surface of the mouthpiece block, wherein a seal material (74) is fixed to both sides of at least either of the base block (42) and the mouthpiece block (43) so as to prevent the entry of dirt such as cutting powder into the tilted sliding surface (50), whereby the defective work pressing down operation by the mouthpiece and the lowering of durability can be prevented, a work supporting table (54) positioning the height of a held work (66) is allowed to be installed on the mouthpiece itself without producing any error of a holding height due to entry of cutting powder and, by using the mouthpiece, the accuracy of height of the work held by the vice can be increased, thus allowing the accuracy of machining to be more increased.
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