Micro-Traction-Drive is made by modified angular contact ball bearings. It is developed for the purpose of transmitting high torque with small drive size. Structure of Micro-Traction-Drive is as follows: The traction force is generated by the thrust preload between inner race and outer race. The retainer connected to the output shaft transmits the driving torque. The retainer is made from copper alloy. The test-runs of Micro-Traction-Drive (outer ring diameter 42mm) have been carried out as tandem stage type (reduction ratio 7.3). The test results demonstrate that Micro-Traction-Drive has been successfully operated at 20,000rpm. Furthermore, material of retainer has been examined. Effect of retainer material on efficiency of Micro-Traction-Drive was revealed.