There is provided a ball mill having a milling chamber into which metal powder is fed. The ball mill is also provided with milling means for milling metal powder into fine metal powder having a particle size less than a predetermined size. When the ball mill operates, a quantity of heat (Q0) is generated in the milling chamber. The milling chamber is cooled by liquid cooling means and gas cooling means according to the present invention. The liquid cooling means causes cooling liquid to flow along the outside wall of the milling chamber to remove a quantity of heat (Q1) during the ball mill operation. The gas cooling means causes cooling gas to flow through the milling chamber to remove a quantity of heat (Q2) during the ball mill operation. The generated quantity of heat (Q0) can be counterbalanced with the sum of the removed quantities of heat (Q1) and (Q2) so as to prevent the inside of the milling chamber from overheating, so that the ball mill can operate in the condition of Q0/V≧0.05 kW/l, where V is the inner volume of the milling chamber.