An overload protecting device assembled in an auger transmission is disclosed as including a worm wheel having its side wall formed with a plurality of wheel-side protruding segments, a boss member engaging with the wheel and fixedly coupled to an auger shaft for non-rotating relationship, and a slide washer having a plurality of wheel-side projections facing the wheel-side protruding segments. When the auger shaft encounters a torque exceeding a given value, the worm wheel and the boss member are enabled to rotate relative to one another. As the wheel-side projections ride on the protruding segments, the slide washer is moved in a direction opposed to the worm wheel. A washer detection switch detects such a movement to produce a detection signal to forcibly stop the operation of an engine.