PURPOSE: To prevent a valve for controlling the number of idle revolutions from generation of knocking by performing duty ratio control so that an electro-magnetic valve mounted in a roundabout passageway of a throttle valve is opened in an amount smaller than that when it is opened to its nearly full extend, for a period from the time when a ignition switch is turned on to the time when the engine operation is started.

CONSTITUTION: An ignition switch is turned on and it is judged whether an engine is at a standstill or not. When the engine is at a standstill, a duty ratio Ds at starting of operation is established at lower value than a duty ratio for controlling an ISC valve to be opened to its nearly full extent. In case that the engine start to rotate, a duty ratio Ds at starting of operation is established at such a value that the ISC valve is opened to its nearly full extent. The ISC valve 16 is controlled by this duty ratio Ds as an output duty ratio Dout.