PURPOSE: To prevent an engine having a less used time from stalling due to insufficient sealing ability, by changing the set value of idle rotational speed into a high value when the set value is decreased in accordance with a rise in engine temperature if the total operating time of the engine or the travel distance thereof is less than a predetermined value.

CONSTITUTION: A control unit 4 sets a desired rotational speed which lowers in accordance with a rise in cooling water temperature during idle operation, in accordance with detection values from a neutral switch 2, a travel distance switch 3, a water temperature 4, a load switch 5, a throttle opening degree sensor 6, a rotational speed sensor 7, etc. That is, for example, if the travel distance is less than twenty thousand miles, the desired set value is set to 1,500, 1,500, 1,500, 1,300, 1,200, 1,000, or 750, while if the travel distance exceeds twenty thousand miles, the desired set value is set to 1,500, 1,500, 1,400, 1,200, 1,000, 750 or 750.