A power transmission device includes a pulley 1 and a hub 2 having an inner hub 21, a rubber damper 22 and an outer hub 23. A hub side engagement portion 24 formed of an elastic material and arranged at either one, or both, of an inner peripheral surface and an outer peripheral surface of the outer hub engages with a pulley side engagement portion 12, which is arranged at a position corresponding to the outer hub, of the pulley in a concavo-convex fitting to thereby constitute a torque transmission structure between the hub and the pulley. The hub side engagement portion and the pulley side engagement portion have a concavo-convex outer shape and mesh with each other. A ring-like dust invasion prevention portion 28 covers a front surface of said concavo-convex fitting.