PURPOSE: To prevent the formation of the groove caused by the wear due to a ball stone on the inside surface of a peripheral wall even if a ball mill is used for long time.

CONSTITUTION: A guide surface 16 is formed so that the ball stone comes not in contact with the inside surface of the peripheral wall 13 and an inside surface of an edge wall 14 simultaneously at the area where the inside surface of the peripheral wall 13 and the inside surface of the edge wall 14 cross each other. In this way, the ball stone does not stay at a prescribed position, and the formation of the groove caused by the wear due to the ball stone is prevented, and the mixing and/or grinding treatment of the material to be treated is executed in uniform quality over a long period of time.