an ion implanter comprises an ion source and a wafer support device having a rotary disk that supports a plurality of wafers thereon and is rotated about its center axis , and capable of being swung alternately in opposite directions . an ion beam emitted by the ion source is projected on the wafers for ion implantation . the wafer support device is supported so that the center of gravity of the wafer support device lies below an axis about which the wafer support device is swung alternately in opposite directions and a component of the gravitational acceleration imparted to the wafer support device acts in the same direction as a force applied to the wafer support device to reverse the same .