PURPOSE: To obtain a sufficient floating power even if the bottom face of a work head is approached to a wafer and to prevent the damage of the wafer by forming a groove communicating the area including the gas discharging port of the work head and a side wall in the mound cutter of a semiconductor wafer.

CONSTITUTION: A groove 9d whose both ends are opened to the side wall of a work head 9 is provided on the bottom face 9a center part of the work head 9 and a gas discharging port 9a is opened inside the groove 9d. A gas discharged from the gas discharging port 9e is passed through the groove 9d and released to the external part from the side wall of a work head 9. The floating power of the work head 9 is therefore decided by an interval (d) (= the depth of the groove 9d + interval c) even if the interval C between the work head 9 and wafer 2 be set minimum and a sufficient floating power can be secured by setting the interval (d) to the extent of about 20μm even if the interval (c) be set in about 3μm, for instance. The posture of the work head 9 can be restored without any bound by the sufficient floating power even if the posture be largely collapsed by the impact of the case of the mound cutting and no wafer is damaged.