A magnet is provided which holds spherical balancers in the central portion of a rotor when the rotation of the rotor is stopped. When rotations of the rotor is started, a larger torque T.sub.0 is transmitted from the rotor to the balancers. Thus, the balancers quickly start rotating. When the rotor rotates at high speed, the centrifugal force separates the balancers from the magnet and moves the same to the outer wall portion of the rotor. At this time, a torque T.sub.0 which is transmitted from the rotor to the balancer is reduced so that the balancers are moved in the circumferential direction. Thus, the automatic aligning operation is performed. The number (n) of the balancers is made to be a value obtained by the following equation:n=180/.theta.+0.5 (all digits to the right of the decimal point are discarded)wherein .theta.: angle of balancer balls disposed in the outer portion, in which .theta.=2sin.sup.-1 {r/(R-r)}R: radius of movable spacer: radius of each balancer ball