To provide a power transmission device to block a power transmission channel of an electric motor and a driven side apparatus at the time when the driven side apparatus is driven by power of an engine.

A first one-way clutch 8 and a first pulley 7 are arranged on an outer peripheral surface of a rotor 4. Additionally, a second oneway clutch 15 and a second pulley 16 are arranged on an outer peripheral surface of a hub 11. The first pulley 7 is connected to a pulley installed on a driveshaft of an engine. The second pulley 16 is connected to a pulley installed on a driveshaft of an electric motor. Motive power is transmitted to an axis of rotation l through the second one-way clutch 15 and the hub 11 as the electric motor is driven. Additionally, motive power is transmitted to the axis of rotation l through the first one-way clutch 8, the rotor 4, an armature 13, a disc spring 12 and the hub 11 as the engine is driven. Furthermore, the second one-way clutch 15 idles at the time when the engine is driven.