To prevent the occurrence of a material to be polished such as glass substrate, to improve the use efficiency of an abrasive and to reduce a cost by surely redispersing abrasive grain agglomerated in the abrasive.

An abrasive used by a polisher is poured from a discharge pipe 5 to a tank 6. A proper number of ultrasonic vibrators 8 are arranged on the peripheral wall 64 of the tank 6 and a bottom wall 65. An ultrasonic wave is shot from these ultrasonic vibrators 8 to the tank 6. Abrasive grain in the abrasive mutually agglomerated by an intermolecular force and made into a relatively large granule in a polishing process receives the vibration energy of ultrasonic wave in the tank 6 and is redispersed as the individual abrasive grain of small diameter.