To automatically stop feed at overload by inclining a projecting contact face of, a slide shaft pushing out steel balls to the recessed groove of a gear in the axial direction of a slide shaft.

A slope 31c contacted with steel balls 30b of a slide shaft meets with the axis of a worm shaft 28 at an angle θ, a distance from the axis to the contact part is set (r), and hence the load W in the axial direction applied on the slide shaft by output torqu T is shown in the formula :W=T/r tan θ. When automatic feed becomes overload, the slide shaft is moved, a projecting part 31b of the slide shaft is moved between the steel balls 30b and 30c, and hence transmission from gears 24, 25, 26 to the worm shaft 28 is intercepted.