PURPOSE: To make it possible to facilitate the lift of a polishing plate in the polishing device of a semi-conductor wafer, by providing a leak valve mechanism which is opened by negative pressure from the contact surface with a workpiece, on the polishing plate.

CONSTITUTION: A wafer 2 is holded on a polishing plate 4 by vacuum absorption force through the vacuum suction passage 5 of the polishing plate and the polishing plate is lowered to a surface plate 1, the wafer 2 is directed downwards, then the vacuum absorption force is released. Further, with adhesive material 6 dropped from a nozzle 7 to the surface plate 1, the surface plate 1 and the polishing plate 4 are rotated with the predetermined speed respectively for performing a mirror surface polishing. After finishing the polishing, the wafer 2 is again absorbed by the vacuum suction passage 5. In this case, as the pressure of the space 8 between the polishing plate 4 and the surface plate 1 becomes lower than air pressure around the polishing plate 4, the valve body 14 of the leak valve mechanism 9 is opened so that the air pressure is invaded from the passage 13 of a inserting part 11 through an air passage 10. Thus, the pressure becomes equal to the air pressure, thereby it is possible to lift the polishing plate 4 and the wafer 2.