An overload protecting device (60) assembled in an auger transmission (18) is disclosed as including a worm wheel (38) having its side wall formed with a plurality of wheel-side protruding segments (64), a boss member (41) engaging with the wheel and fixedly coupled to an auger shaft (22) for non-rotating relationship, and a slide washer (45) having a plurality of wheel-side projections (67) 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 (53) detects such a movement to produce a detection signal to forcibly stop the operation of an engine (15).