A power transmission device includes: a pulley 1 rotatably attached to a casing 9; a hub 2 connected to the pulley 1 by an engagement of protrusions with recesses; and a power shutoff member 3 interposed between the hub 2 and a rotary shaft 4. The power shutoff member and the rotary shaft 4 are joined to each other by means of screwing. When a water intrusion path formed between a hub side seating face of the hub and a shaft side seating face 44 of the rotary shaft is formed into a labyrinth structure, water is prevented from entering the seating face. An engagement portion 35 on the power shutoff member side is engaged with a hub side engagement portion 21a so that it can be accommodated in the hub side engagement portion 21a, and the power shutoff member can be rotated together when the hub is rotated. An engagement gap formed between a power shutoff member side engagement portion and a hub side engagement portion in the radial direction perpendicular to an axis of the rotary shaft is in a range from 0.001 mm to 15% of a diameter of a circumscribed circle of the power shutoff member side engagement portion.