PURPOSE: To prevent over lean and over rich of mixture in the captioned device which supplies auxiliary air to an intake passage of a throttle valve lower stream at the beginning of deceleration, by controlling corrective air supply amount according to the detected result of intake air density.

CONSTITUTION: An auxiliary air supply passage 4 for supplying auxiliary air to the lower stream of a throttle value 3 is connected to an intake passage 2 of an engine, and an auxiliary engine control valve 5 is interposed in this auxiliary air supply passage 4. This auxiliary air control valve 5 is driven and controlled by intake air negative pressure so that it is opened at the beginning of deceleration when intake air negative pressure rapidly drops. In this case, a detecting means 6 for detecting the density state of intake air supplied to the engine by, for example, intake air temperature, atmospheric pressure and so forth is provided. And correction is carried out to reduce or stop the opening operation amount of the auxiliary air control valve 5 at the beginning of deceleration when the intake air density is low compared that the intake air density is high, by a correcting means 7 based on the output of a detecting device 6.