A power transmission device includes a pulley (6) serving as a rotation member, an elastic member serving as a power transmission member, a fixing screw extending through these, and a nut that sandwiches the pulley (6) and the elastic member in cooperation with the fixing screw. The power transmission device includes a nut holder (25) attached to the pulley (6) via a hooking structure. The hooking structure includes a through hole (14) of the pulley (6), a hook (13) of the nut holder (25), which can be inserted into the through hole (14), and an elastic piece (33) located near the hook (13). When the nut holder (25) is rotated in one of rotation directions with respect to the pulley (6) in a state in which the hook (13) is inserted into the through hole (14), a hooking piece (36) of the hook (13) comes into contact with the pulley (6), and a pawl portion (43) of the elastic piece (33) engages with the opening edge of the through hole (14). It is possible to provide a power transmission device that employs a structure capable of easily attaching the nut holder and prevents the nut holder from being disengaged by a pressing force that presses the fixing screw against a nut.