A power transmission device includes an electromagnet, a rotor, an armature and a hub. The hub couples the armature to a shaft of a drive-subject device. The hub includes: an outer hub that is coupled to the armature; a boss portion that is coupled to the shaft; an inner side plate-shaped portion that extends from the boss portion toward a radially outer side; and an inner plate that is placed between the outer hub and the inner side plate-shaped portion. The inner side plate-shaped portion and the inner plate are formed integrally in one piece. At least one of the inner side plate-shaped portion and the inner plate is formed by a member that is configured to deform or melt with heat generated by friction between the rotor and the armature when the shaft of the drive-subject device is locked at a time of energizing the electromagnet.