Abstract: A tool holder has a shank, a flange and a tool holding portion. From its front to rear end, the shank has on its outer periphery a first cylindrical surface, a tapered surface, and a second cylindrical surface. A tapered sleeve is mounted on the shank. Belleville springs are mounted between the large-diameter end of the tapered sleeve and the flange of the tool holder. The tapered sleeve is formed with a tapered outer surface adapted to be brought into close contact with a tapered inner surface of a bore formed in the spindle, and a cylindrical inner surface adapted to be brought into close contact with the second cylindrical surface of the shank. If a gap forms between the spindle and the tapered sleeve when the spindle is rotating at a high speed, the tapered sleeve is urged by the belleville springs in such a direction that the gap disappears, thereby maintaining a centripetal tendency and rigidity.