The cutting force of the small diameter drill is not reported, although it is a very important factor in the study on its cutting performance. So, the auother devised a dynamometer for this very small torque and thrust to the constant feed of the spindle. By using this dynamometer, the small cutting force, the torque 0.008 cm-kg, the thrust 0.2 kg of 0.2 mmφ drill can be measured at the same time. At first, the mechanism of this dynamometer is described and by using this dynamometer, it has been found that the work-hardning of the surface which is caused by the machining depends on the cutting force, especially the thrust, and this work-hardning is a factor of the cutting performance, and the recipro-machining with a sand paper is preferable to the finish machining.