To provide a power transmission device capable of reducing operation force when cutting off the power transmission.

The power transmission device is provided for switching mutual projection parts 12 and 22 mutually opposed by a permanent magnet 31 to a coupled state of being magnetically coupled and an uncoupled state of being magnetically uncoupled by the movement of a magnetic coupling control body 30. An uncoupling control body 140 is arranged in a part being the opposite side of a pulley side rotary body 10 and a shaft side rotary body 20 to the magnetic coupling control body 30. When moving the magnetic coupling control body 30 along the rotational axis from the coupled state, these mutual magnetic coupling control body 30 and uncoupling control body 140 are magnetically coupled by the permanent magnet 31.