PURPOSE: To improve a pulverization treatment capacity by providing a rotary cylinder with plural gas blow ports in the positions facing a slurry during the rotation of a ball mill apart spaced intervals in its circumferential direction and providing these gas blow ports with porous bodies allowing the permeation of only the gas.

CONSTITUTION: Balls 1B as pulverizing media are fed into the rotary cylinder 1A. The rotary cylinder 1A is provided with the plural gas blow ports 2, 2a to 2c in the positions facing the slurry S at least during the rotation of the ball mill apart the spaced intervals in its circumferential direction. The gas is blown to the ball mill from these gas blow ports during the rotation of the mill. The gas blow ports facing the slurry S are provided with the porous bodies which allow the permeation of the gas and do not allow the permeation of the liquid of the slurry medium. Consequently, the pulverization treatment capacity per unit time is greatly improved and the backflow of the slurry medium liquid from the blow ports is prevented.