The present invention relates to a governor device for an internal combustion engine where a portion of the engine output is taken off as driving force for vehicle mounted accessory equipment which requires constant speed rotation with changes in load. By controlling the intake air quantity passing through a bypass passage which bypasses a mixing device for mixing fuel and intake air at a predetermined ratio, controlling a sub-throttle valve provided in an intake passage in series with a main throttle valve, and controlling a supply quantity of auxiliary gas fuel so that a detected air-fuel ratio is constant, the engine output is increase/decrease controlled in spite of changes in load in accessory equipment, to give constant engine rotational speed and good operability of an auxiliary power take-off device.