An automatic ball balancer is a unique vibration suppression device for rotor systems. Theoretically, two balls in a cylindrical chamber of the rotor are located at the optimal positions on the opposite side to the unbalance and cancel the unbalance automatically in the super-critical speed range. However, this device is not used widely due to two malfunctions. One is the influence of friction. Due to the inevitable friction between the balls and the inside wall of the channel, the balls stop near the optimal positions and do not balance the rotor perfectly. The other is the self-excited oscillation which occurs near and above the major critical speed. The objectives of the present paper are to clarify the fundamental characteristics of a ball balancer and to introduce some simple methods to eliminate these malfunctions.