PURPOSE: To provide a nucleic acid preparing device which has a large throughput.

CONSTITUTION: Relating to a device 10 based on centrifugation under perfect automatic computer control for preparing DNA, while connected to a personal computer, all functions and operations of the device are controlled by the computer. The device suitable for used together with trace-amount titration plates 78, 80 and 82, a liquid is added in a well 14 of centrifugal separator 12 and the liquid is removed from the well, and then the liquid is mixed by the well, and the liquid is moved from one well to another, and then the trace-amount titration plate is loaded on the centrifugal separator. The device comprises an oscillating bucket 20 which allows access from above and an automatic feedback rotor positioning system of computer control.