PURPOSE: To maintain the engine rotation after start at the optimal level by controlling a control valve in bypath detouring a throttle valve with correspondence to the results of comparison between a pre-stored target rotation and actual engine rotation.

CONSTITUTION: On the basis of output signals from starter switch sensor 1 and an engine rotation counter 2 for counting pulses produced fromm TDC sensor 5, start decision means 3 will decide whether it is under starting or after starting. Upon decision of after starting, the target rotation under idling set as a function of cooling water temperature sensed through a water temperature sensor 4 is read out from a target rotation memory 6 and compared with the engine rotation counted by said counter 2 through a comparator 19. Then on the basis of the deviation of rotation and the variation rate detected through engine rotation variation detecting means 26, PID operating means 10 will operate the proportional term, the integrating term and the differentiating term thus to perform feedback control of supply current to the linear solenoid 16 of control valve.