To adjust the rotation balance of an outside rotary member by restraining the effect to the formation of a magnetic path.

The power transmission device comprises inside and outside rotary members supported by the member so as to be rotatable with each other, a clutch part which connects and disconnects the inside and outside rotary members, and an electric magnetic actuator to connect and disconnect the clutch part. A balancer is fixed in the outside rotary member to adjust the rotary balance, and the balancer serves as a balance weight fixed to the outer peripheral surface of the outside rotary member.