To provide a power transmission device, in which the time for replacement of a belt is detected with a high degree of accuracy.

The power transmission device 1 includes a timing belt 13 that is wound around a crankshaft pulley 10, an intake camshaft pulley 11, and an exhaust camshaft pulley 12, and on which magnetic resin is applied to a surface of a tooth cloth. The device includes: a magnetic detection circuit 16 for detecting magnetism of the timing belt 13; and an ECU 100 for determining whether the timing belt 13 becomes due for replacement, on the basis of the magnitude of magnetism of the timing belt 13 detected by the magnetic detection circuit 16.