An automatic speed reduction ratio switching apparatus is equipped with a sun gear connected to an input shaft, first and second carriers connected to a feed screw shaft, planetary gears meshed with the sun gear and which are mounted rotatably in the first and second carriers, and an internal gear meshed with the planetary gears and which is capable of movement in an axial direction. A packing member is arranged between the internal gear and the second carrier, for regulating movement in the axial direction of the internal gear, while enabling movement of the internal gear in the axial direction when a thrust force is applied at or above a predetermined value.