To reduce a running cost of a lift-feed polishing machine by judging appropriate exchange timing of a polishing material for pachinko balls.

It is characterized that a running cost of the polishing machine can be reduced by easily judging appropriate exchange timing of the polishing material by everybody and avoiding a cause of uneconomical efficiency and a reduction of a polishing ability. A mixing polishing material is formed by mixing a contrast pellet 3b colored to a color assumed that it arrives at a usage limit in a usual polishing material 3a and when the polishing material is repeatedly used and soiling is advanced, appropriate exchange timing is judged from a reduction degree of a contrast of both. For example, as shown in Fig. (a) and Fig. (b)-(c), it is judged that the polishing material arrives at the usage limit and an exchange is required regarding the polishing material in which a color change is advanced and a discrimination of both becomes difficult by a reduction of the contrast.