A cold-engine operation detector is provided for producing a cold-engine signal when engine temperature is lower than a predetermined temperature, and an idle engine speed detector is provided. A dead zone for input of engine speed with respect to desired engine speed in accordance with engine temperature is stored in a memory. Width of the dead zone is provided to increase with decrease of engine temperature. A range of the dead zone in accordance with engine temperature is derived from the memory. When actual engine speed is out of a derived range of the dead zone, an actuator provided in an idle speed control system is operated for controlling the actual engine speed to the desired engine speed.