A power transmission device(10,50) comprising: a first input shaft and a second input shaft coupled to a first motor and a second motor, respectively, and disposed on the same axis; and a first speed reduction mechanism transmitting the rotation of the first input shaft to an output shaft via an intermediate shaft and a gear; characterized in that the power transmission system (10,50) further comprises: a second speed reduction mechanism transmitting the rotation of the second input shaft to the output shaft via the intermediate shaft at a speed reduction ratio lower than the speed reduction ratio of the first speed reduction mechanism; and a first clutch consisting of a one-way clutch interposed between the gear and the intermediate shaft on which the gear is disposed, wherein, when the power transmission device (10,50) drives the first motor so that the rotation speed of the second gear is relatively higher than the rotation speed of the intermediate shaft, the first clutch is connected and the first motor outputs the torque to the output shaft via the first speed reduction mechanism, and the first clutch is disconnected when the speed of rotation of the second gear is relatively lower than the speed of rotation of the intermediate shaft. a first input shaft and a second input shaft coupled to a first motor and a second motor, respectively, and disposed on the same axis; and a first speed reduction mechanism transmitting the rotation of the first input shaft to an output shaft via an intermediate shaft and a gear; characterized in that the power transmission system (10,50) further comprises: a second speed reduction mechanism transmitting the rotation of the second input shaft to the output shaft via the intermediate shaft at a speed reduction ratio lower than the speed reduction ratio of the first speed reduction mechanism; and a first clutch consisting of a one-way clutch interposed between the gear and the intermediate shaft on which the gear is disposed, wherein, when the power transmission device (10,50) drives the first motor so that the rotation speed of the second gear is relatively higher than the rotation speed of the intermediate shaft, the first clutch is connected and the first motor outputs the torque to the output shaft via the first speed reduction mechanism, and the first clutch is disconnected when the speed of rotation of the second gear is relatively lower than the speed of rotation of the intermediate shaft. a second speed reduction mechanism transmitting the rotation of the second input shaft to the output shaft via the intermediate shaft at a speed reduction ratio lower than the speed reduction ratio of the first speed reduction mechanism; and a first clutch consisting of a one-way clutch interposed between the gear and the intermediate shaft on which the gear is disposed, wherein, when the power transmission device (10,50) drives the first motor so that the rotation speed of the second gear is relatively higher than the rotation speed of the intermediate shaft, the first clutch is connected and the first motor outputs the torque to the output shaft via the first speed reduction mechanism, and the first clutch is disconnected when the speed of rotation of the second gear is relatively lower than the speed of rotation of the intermediate shaft.