PURPOSE: To relieve impact when meshing of gears is released, perform a uniform and smooth return action, and make external influence such as rust hard to be exerted by arranging a slide gear return spring in a housing on the circumference between fixing bolts of at least two places.

CONSTITUTION: A housing part on the circumference of at least two places between fixing bolts 8 and 8 in a housing 9 fixed to a wheel shaft hub 13 is padded in the shaft direction, and is formed as a padding part 10, and a coil shaped slide gear return spring 5 is installed in a shaft directional hollow part 11 in the padding part 10. When a space between the fixing bolts is effectively used, the slide gear return spring 5 having a long stroke in the shaft direction can be installed in this part, and impact when meshing of gears is released can be effectively relieved, and the slide gear return spring 5 presses a retainer 4 at least in two places, and not only a uniform and smooth return action works well but also a central position of a diaphragm 6 and the retainer 4 can be easily adopted.