Differential rotary to linear conversion mechanism with three precision ball screws and three spur gears were designed in order to evaluate the positioning accuracy and the input-output characteristics under the applied output load. In the positioning accuracy test, the positioning accuracy of this mechanism was almost equivalent to the accuracy of the precision ball screw employed in the test model. In the input torque-output load conversion test, the output load per unit input torque of this mechanism was larger than that of the feed screw mechanism. These results revealed that the differential rotary to linear conversion mechanism with three precision ball screws and three spur gears has the characteristics of both high positioning accuracy and excellent input-output conversion efficiency.