A damper member (80) is provided between the worm wheel (39) and the output member (70), and elastically deformed by a relative rotation of the worm wheel (39) and the output member (70), a gap portion (SP) is provided between the worm wheel (39) and the output member (70), a portion (DP) bulged by elastic deformation of the damper member (80) is housed in the gap portion (SP), the portion (DP) bulged by elastic deformation of the damper member (80) does not push the output member (70) away from the worm wheel (39). Therefore, although it has the damper member (80), it is not necessary to fix the output member (70) so as to prevent the output member (70) from being moved in an axial direction of a support shaft by using a fixing member on the basis of a conventional manner, thereby making it possible to reduce the intensity of the support shaft, and consequently, it is possible to realize excellent shock resistance and silence, and cost reduction.