To surely prevent the reverse rotation of a motor due to reaction from a load side during powering the motor off without separating a driving shaft.

This reverse rotation preventing mechanism 120 holds a stopper 124 in a body 122 movably in the direction of leaving the motor driving shaft 110 at a predetermined distance, and energizes the stopper 124 toward the motor driving shaft 110 with a pressure spring 126. A sprag 128 functioning as a lock member is arranged between the motor driving shaft 110 and the stopper 124 so as to be energized to a neutral position by a neutral spring 130. The sprag 128 maximizes the thrust of the pressure spring 126 on the motor driving shaft 110 at the neutral position and reduces the thrust on the motor driving shaft 110 as leaving the neutral position. The spring load of the pressure spring 126 is set so that friction torque on the motor driving shaft 110 at the neutral position of the sprag 128 is greater than reverse torque from the load side and smaller than the driving torque of the motor.