PURPOSE: To prevent occurrence of a drop down phenomenon in an engine speed at deceleration in idle operation, by controlling a throttle opening in accordance with a change of actual idle speed.

CONSTITUTION: An arithmetic means 100 is constituted by an arithmetic unit 80, outputting a proportional value to a change of actual speed (a) in a range with output of the actual speed (a) from a speed sensor 30 at least a certain level higher than a target speed (b), and an arithmetic unit 40, comparing the actual speed (a), detected by the speed sensor 30, with the target speed (b) and outputting an integrated value of the deviation (c). Then at deceleration of an internal- combustion engine 10 after being raced, its actual speed (a) is dropped, however, the arithmetic unit 80 genetates an output value in proportion to the drop of the actual speed (a), and a target control position (e) rises to increase an actual control position (d) of a throttle stopper 20, thus opening of a throttle valve 11 is increased. As a result, a dropping speed of the actual speed (a) can be decreased.