To generate braking force for preventing rotation due to external force corresponding to running-in of a transmission system.

On the axial end face 4a of a rotating shaft 4 of a motor, a first projecting is projected on the center of rotation coaxially, and a second annular projecting part 12 coaxially enveloping the first projecting part is projected. The height in the projection direction of the first projecting part is set larger than that of the second projecting part. The first projecting part is abutted to the opposite surface until the first projecting part wears away for a predetermined amount, the braking force against the external force in the rotating shaft is generated by the frictional resistance of the first projecting part to the opposite surface, and after the first projecting part wears away as described above, the braking force to the rotating shaft is generated by the frictional resistance caused by the positive abutting of the second projecting part to the opposite surface. Thus, the positions (radii) and areas of the respective projecting parts can be set corresponding to the respective cases so that the static frictional force is small at the initial state until running-in of the transmission system, and after the running-in is achieved by operation, the static frictional force becomes larger.

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