PURPOSE: To control automatically the rotational speed of an engine by control a throttle valve in the closing direction for controlling the rotational speed of the engine to the highest allowable one when said rotational speed exceeds the highest allowable one according to the throttle operation.

CONSTITUTION: The rotational speed of a flywheel magnet 42 of an engine body 10 is detected by an engine-rotational-speed sensor 43 so that the detection signal is sent to a frequency-voltage converter 45 which converts the detection signal to the detection value Vx of voltage. And the detection value Vx is compared with the set value Vs of the highest allowable rotational speed of a setter 46 in a comparator 47. When the detection value Vx exceeds the set value Vs as a result of the comparison, a drive motor 38 is driven by the output signal of the comparator 47 so that a throttle valve of the engine is controlled in the closing direction until the rotational speed of the engine reaches the set value Vs.