PURPOSE: To renew balls surely and efficiently without relying upon any manual work, by a method wherein a detouring pipe, interposed with a vibration type ball selector and a ball metering device, is provided while respective valves are operated in adequate time series by the instruction of a ball monitoring device.

CONSTITUTION: The maintenance of balls is started by a ball monitoring device 7, detecting the shortage in the number of effective balls, and a ball flap 10, an inlet valve 11, an outlet valve 12, a ball discharging valve 14, a ball supplying valve 20, a ball throwing-in valve 22 and a ball retriever drain valve 23 are opened and/or closed in accordance with a predetermined sequence to stop the circulation of the balls temporarily, send the using balls 3 into a vibrating type ball selector 16 through a ball retriever 9 and throw effective balls into a ball metering device 21 in order to reutilize effective balls while discharging faulty balls in the ball selector 16. The number of shortage of the balls 3, known by the result of metering, is corrected by throwing new balls from a ball supplementing chamber 18 into the ball metering device 21 while counting the number of new balls. The balls 3 are circulated again through the system together with cooling water after the balls are thrown into the ball retriever 9.