In order to minimize unwanted corotation, a gear support member is in the shape
of a flat disk and has an outer diameter greater than the pitch circle of a ring
gear. Further, a surface level of lubricating oil stored in a casing is set to
be below the ring gear, but above the gear support member. Furthermore, a resilient
clip is resiliently fitted on an output shaft and gear support member, and the
resilient clip has at least one end supported by the casing. In addition, the casing
has, on its bottom, a frictional surface for sliding contact with the lower surface
of the gear support member.
