PURPOSE: To shorten the time required for warming up an engine and at the same time to keep the performance of the engine, by providing a means for detecting the temperature of engine cooling water, a means for detecting running conditions and a control means for operating an actuator.

CONSTITUTION: In case of warming up an engine while keeping a vehicle at rest, an aimed idling speed is calculated at an aimed idling speed calculating circuit 11 on the basis of the output signal of a water-temperature sensor 12. The aimed idling speed thus obtained is furnished to a comparison circuit 16, and an idling- speed control valve 7 is moved to its opening direction by a driving circuit 19, whereby the idling speed is raised and the time required for warming up the engine is shortened. In case of running a vehicle while warming up the engine, on the other hand, a correction circuit 13 is set into operation by a gear position sensor 14 via a judging circuit 15, whereby the idling speed can be kept at an ordinary level and normal engine performance can be maintained. Thus, it is enabled to shorten the time required for warming up an engine and at the same time to securely maintain the performance of the engine.