To reduce useless exchange and to eliminate a uselessness in terms of economy and labor by previously setting the exchange period of abrasives, comparing the number of used Pachinko (Japanese pinball game) balls after usage for a game with a set value and grasping the exchange period.

When a numerical value is inputted from a keyboard 67, the value is transmitted to a RAM 64 and stored as a prescribed one, the stored prescribed number value is compared with the used Pachinko balls by a CPU 60, counting is continued unless the result is more than the prescribed number, an announcing signal is outputted when the result becomes more than the prescribed one and it is displayed in a display device 70. The attention of an operator is called by issuing alarm to announce that the result becomes more than the prescribed numerical value together with the display, and then, a command is transmitted to a motor. Then, an opening/closing valve is opened and dirty abrasive grains inside a changeover chamber in a flow-down direction are discharged to an abrasive grains discharge path.