PURPOSE: To compensate unbalance of a whetstone by introducing compressed air alternately into each of chambers and controlling quantity of liquid distributed in the tank.

CONSTITUTION: A rotary four direction compressed air distribution device 8 is fixed to the housing 1 at the opposite side of the cover 7. Each of fiur direction passways is connected through passages 9 at the center of the water-tight housing 1 to one of the chambers 2, so that the compressed air is introduced into one of the chambers 2 compressing the tank 3. The liquid within the compressed tank, then, flows through a capillary tube connecting two tanks to other side of the tank. Thus, unbalance of the whetstone is compensated by the liquid quantity distribution in the tank 3.