PURPOSE: To raise an operational feeling, by setting up increase and decrease rate of intake air quantity in load changeover as a function of controlled engine speed target when the idle engine speed is controlled by regulating intake air quantity in the by-pass conduit which by-passes a throttle valve.

CONSTITUTION: An air control valve 18 is provided in a by-pass air intake conduit 16 which by-passes the throttle valve 14 of an air intake conduit 12. The opening of valve 18 is controlled in response to the negative pressure introduced into the diaphragm chamber 18a through the conduit 24 interposing a negative pressure switching valve 26. The negative pressure switching valve 26 is adapted to control the increase and decrease of intake air flow by a control circuit 28 in response to the degree of an engine load. The control circuit 28 can set up the increase and decrease values of the intake air flow rate as a function of either controlled engine speed target with a load or controlled engine speed target with no load. The engine speed is, thereby, prevented from over-shooting or under-shooting when the load is changed.