PURPOSE: Since the input rotary part and two gears meshed in the control rotating unit each other revolve at the opposite direction the reduction device can prevent the safety accident by the unnecessary driving force. CONSTITUTION: The sun gear(S) is used as the driving input rotating unit(11) of the principal axis delivered the first rotation power source through the principal axis. It is combined with the gear having the gear ratio which the sun gear constants. The ring gear is used as the shift control rotating unit(12) of the principal axis.
