Planetary gear reducer is composed of Sun gear, Planetary gear and Ring gear. As its power transmission rate per unit volume is very high, it can deliver power compared to the same sized general gear reducer.

Input shaft and output shaft can have the same center, and the power transmission efficiency is high.

As this has wide range of reduction gear ratio and high power transmission efficiency values, this is used for diverse purposes from ordinary industrial reduction gear to machine tools, vehicles and airplanes.