PURPOSE: To always maintain driving precision of an idle rotational speed control valve (ISC valve) properly by means of an inexpensive constitution requiring no dedicated temperature sensor or no special structure.

CONSTITUTION: A target idle rotational speed is computed on the basis of an output from a cooling water temperature sensor 23 in an engine, and from an engine rotational speed detected by means of a rotational speed sensor 22 in the engine and the target idle rotational speed, a deviation in rotational speed is computed each time. On the other hand, a basic duty value for an ISC valve driving pulse is computed on the basis of the deviation in rotational speed and the like, and solenoid coil self-heat generating temperature of an ISC valve 18 is assumed on the basis of the basic duty value. The assumed coil self-heat generating temperature is added to the output of an intake temperature sensor 14, and coil temperature of the solenoid coil is computed, and then, on the basis of the computed coil temperature, the basic duty value is corrected sequentially.