PURPOSE: To aim at preventing engine-stall, by once increasing the amount of intake-air when the rotational speed of an engine drops, largely in comparison with a desired idle speed, and by thereafter decreasing, at once, the amount of intake-air.

CONSTITUTION: An idle speed control device operates in such a way that an actual idle speed detected by a rotational speed detector 11 is compared with a desired idle speed set in accordance with the operating condition of an engine, by means of a microcomputer 13, and an actuator 9 is driven under control in accordance with the deviation due to the comparison, to control the opening degree of a throttle valve 7. At this time, if the rotational speed of the engine is below a first predetermined rotational speed which is lower than the desired rotational speed, the microcomputer 13 operates the actuator 9 by a first predetermined amount in order to aim at increasing the amount of intake-air. Thereafter, in order to immediately reduce the amount of intake-air, the actuator 9 is driven under control by a second predetermined amount which is set to be less than variations in the first predetermined amount.