A power transmission unit that can multiply torque of a prime mover to be delivered to a differential gear unit is provided. Torque of a drive motor is delivered to planetary gear units through a driven gear. A first ring gear of the first planetary gear unit is meshed with a first supplemental gear, and a second ring gear of the second planetary gear unit is meshed with a second supplemental gear. The first supplemental gear and the second supplemental gear are connected to each other through a reaction gear unit to reverse the torque applied to the first ring gear to be transmitted to the second ring gear.