To provide a power transmission device wherein fatigue strength is maintained at the same level as that of the conventional power transmission device, and static breakdown torque is reduced, so that a rupture portion can be ruptured before a power source is stopped even when the output of the power source is small.

A power transmission member which is secured to the drive shaft of a compressor and transmits rotational power from a drive side rotator rotated by a drive source, includes a hub 4 in which a plurality of holes arranged in the circumferential direction so as to surround the drive shaft are formed and thereby, rupture portions are configured between the holes by bridge parts 22 each having a prescribed length in the radial direction. Grooves 25 for reducing the breakdown torque are provided at portions excluding portions at which the stress is maximum in usual usage torque range (elastic deformation range), on the surface of each bridge portion 22 of the hub 4.