PURPOSE:To always maintain necessary number of balls needed to the process of cleaning balls in a ball circulating system, by counting the number of balls to be circulated, the number of worn out balls, and that of the ones to be newly replaced, and by controlling respective valve based on the counting results. CONSTITUTION:The ball circulating pump 7, a collector 8, a selector 20, a water soaker 21, and valves 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, and 60 in front and in the rear of a vacuum pump 22 are controlled, by providing counters 12a, 12b and 12c respectively to each side of the outlet of pump 7 to circulate ball circulating water, of ball selector 20, and of ball supplier 23, by counting the number of balls to be circulated, of worn out balls and of balls to be newly replaced by the counters 12a, 12b and 12c, and by putting the results measured into the processor 100 to calculate the number of balls necessary for cleaning. By these units of equipment, the necessary number of balls needed to the cleaning process in the ball circulating system can always be maintained.