A power transmission device includes a motor, a first planetary reduction gear connected downstream of the motor, a second planetary reduction gear connected downstream of the first planetary reduction gear, a differential gear connected downstream of the second planetary reduction gear, a drive shaft connected downstream of the differential gear, and a case member. The drive shaft penetrates an inner circumference of a rotor of the motor, an inner circumference of a sun gear of the first planetary reduction gear, and an inner circumference of a sun gear of the second planetary reduction gear. A portion of the differential case positioned between the first planetary reduction gear and the second planetary reduction gear is supported at an inner circumference side of a portion of the case member with a first bearing being interposed.