To provide a torque transmission device 1 for starting an engine easy to be manufactured, capable of cutting cost and preventing an outer 13 from being deformed when transmitting torque.

The outer 13 of a clutch 6 is provided separately from a first rotor 3 and the outer circumferential surface of a press-in part 13a integrally provided with the outer 13 is pressed in and fixed with a predetermined fastening margin at the inner circumferential surface of a press-in hole formed at the first rotor 3. By this structure, since the outer circumferential surface of the press-in part 13a is pressed in to the inner circumferential surface of the press-in hole part with the predetermined fastening margin, it is possible to prevent the outer 13 from being deformed to an outer circumferential side by a load applied from an engaging piece 15 when the torque is transmitted by the clutch 6. As a result, the leakage of grease filled in the clutch 6 can be prevented, the deformation of the engaging piece 15 can be prevented, and it is possible to surely transmit the torque in a stable state.