PURPOSE: To provide an idle speed control device for engine which can control the idle speed without requiring an exclusive structure and parts such as an ISC(idle speed control) valve.

CONSTITUTION: An engine idle speed control device is provided with an intake air control valve 17 to control variably an area of an intake air passage 7 so as to generate a vertical vortex or a horizontal vortex in an intake air flow introduced in a cylinder 2a, an opening detecting means 21 to detect opening of the intake air control valve 17 and an engine speed detecting means to detect engine speed. In this case, an opening control means (ECU) 21 is arranged to control respectively opening of the intake air control valve 17 to the side where the vertical vortex or the horizontal vortex is strengthened when the engine speed is lower than target idle speed and to the side where the vortex is weakened when the engine speed is higher than the target idle speed.