A first planetary gear unit is arranged to convert an input rotation from an input shaft to an output rotation whose speed is lower than that of the input rotation. A first unit including second and third planetary gear units is arranged between the first planetary gear unit and the output shaft. The first unit manages the first planetary gear unit's output rotation and provides the output shaft with seven types of rotation which correspond to 6-forward speed and one reverse positions. A second unit is arranged between the first planetary gear unit and the first unit to manage a power transmission therebetween. One of the second and third planetary gear units is of a double ring type, each being meshed with the sun gear and inside and outside ring gears, and a pinion carrier carrying the pinions. The pinion carrier is connected to the output shaft to rotate therewith.