PURPOSE: To prevent gear lubricating oil from externally flowing out and entering into a main motor bearing by adding a small-diameter annular wall inside a felt retainer metal.

CONSTITUTION: The position of a boundary gap 16 between a bearing stopper 14 (rotary unit) and a bearing box cover 15 (nonrotary unit) of a main motor bearing is disposed near the engaged section of a gear, and scattered oil is fed through the gap 16 into the bearing or fed out from a felt seal in the retainer metal 6 of a gear box. Then, an annular wall 22 which can become a protection wall is provided on the gear engaging face inside of the metal 6, and reduced in diameter with respect to the main motor axial center. Thus, it can prevent the scattered oil from externally flowing out and entering into the bearing.