In an engine of a working machine having no governor mechanism, the engine revolution speed is controlled such that the revolution speed can stably be maintained irrespective of the variation in load without adjusting the throttle opening to exploit the full potential of the engine. Since an engine 1 does not have the governor mechanism, an operator adjusts the throttle opening and controls the engine revolution speed. A map in which ignition timing (amount of angle of lead) is set by means of functions of the engine revolution speed is stored in an ignition timing map 30. The map 30 is set such that the engine revolution speed is equal to or higher than a value to be controlled (e.g. 7000 rpm), and the ignition timing is lagged as compared with a case in which the engine revolution speed is lower than the value to be controlled.