The present power transmission mechanism comprises: a first rotation member mounted integrally rotatably on the rotation shaft of a drive apparatus; a second rotation member rotatable integrally with or relative to the first rotation member; an elastic member interposed between the first and second rotation members which, when the first and second rotation members rotate relative to each other, absorbs a rotation difference between them; and, a switching portion which includes a mass member mounted on one of the first and second rotation members and rotatable integrally therewith, and an energizing member for energizing the mass member in a direction where it can be contacted with the other of the first and second rotation members. In the switching portion, when the number of rotations of the rotation shaft is a given number of rotations or less, the mass member is contacted with the other rotation member due to the energizing force of the energizing member to generate a frictional force between their contacted surfaces, thereby preventing the relative rotation between the first and second rotation members. And, when the number of rotations of the rotation shaft exceeds a given number of rotations, due to a centrifugal force generated from the rotation of the rotation shaft, the mass member is shifted against the energizing force of the energizing member and is separated from the other rotation member, thereby allowing the relative rotation between the first and second rotation members through the elastic member.