We propose a new robot actuator, especially a robot actuator gear which has a very effective feature of backdrivability. We show the study of the new definition of backdrivability of an actuator gear which has the quantitative definition. From this definition we propose the method of making the gear which has a good backdrivability. Based on this method, the actuator gear was developed and we show the result from the experiment. The comparisons with the other types of actuator gear which are Harmonic drive gear and normal planetary gear are described. Finally the comparison has proved that the developed actuator gear has very effective backdrivability.