PURPOSE: To renew balls without relying upon any manual work, by a method wherein a ball throwing device is arranged above a ball retrieving device so as to be communicated while respective valves are operated so as to be opened and/or closed by the instruction of a ball monitoring device in an adequate time series.

CONSTITUTION: The maintenance of balls is started by a ball monitoring device 7, detecting the shortage in the number of effective balls, and valves, such as a ball flap 10, an outlet valve 12, a ball discharging valve 13, a ball throwing valve 18 and an inlet valve 11, are opened and/or closed in accordance with a predetermined sequence to stop the circulation of the balls temporarily and collect used balls to discharge them through a provided ball retrieving device 9. All of operations from throwing a predetermined amount of new balls from a ball throwing device 16 until the re-circulation of the balls is started may be effected automatically.