PURPOSE: To stabilize idling engine speed control by providing a heating means for heating the coil of a solenoid valve to an engine cooling water temp. and making a bypass air flow rate characteristic by said solenoid valve correspond to said cooling water temp.

CONSTITUTION: A feedback control means D controls the duty ratio of a solenoid valve C based on a target idling engine speed and a feedback coefficient which are set according to an engine cooling water temp. by a means F, varying the flow rate of air flowing in a bypass B which bypasses a throttle valve A to carry out feedback control to the target idling engine speed. A heating means E for heating a coil to the engine cooling water temp., i.e., a water jacket for introducing the cooling water from an engine to around the coil is provided on this solenoid valve C, to make its air flow rate characteristic correspond to the cooling water temp. Thereby, the time at which a target idling engine speed is attained is reduced, carrying out stable idling engine speed control.