differential change rate of the deceleration control system 16 if the foregoing conditions are satisfied then the control portion 18 decides the duty rate of the control signal applied to the deceleration control system 16 such that the duty rate corresponds to the reductive change rate of engine speed and increases the air intake rate specifically the control portion 18 as shown in fig3 actuates the deceleration control system 16 only when all of the illustrated conditions are satisfied as discussed below the engine cooling water temperature measured by the cooling water temperature sensor 20 must be equal to or higher than the predetermined water temperature that is the accelerator switch 28 must be in its inactive off state such that the engine is decelerating e g the idle switch 24 is in its active on state that is the engine speed ne must be equal to or lower than the actuation speed dpne of the deceleration control system that is the differential change u03b4ne of the engine speed must be negative such that the engine is decelerating that is the reductive change rate u03b4ne time of enqine speed as shown in fig5 must be equal to or larger than the actuation differential change rate dpot of the deceleration control system 16 that is when the deceleration control system 16 is actuated the control portion 18 as shown in fig5 decides the control signal duty rate of the deceleration control system 16 in such a manner as to be in correspondence with the reductive change rate u03b4ne time of engine.