A control apparatus for use in an internal combustion engine system of a motor vehicle equipped with an automatic transmission having therein a lock-up mechanism. There is included a vehicle resonance reducing section for controlling an opening degree of a throttle valve when the motor vehicle is in an acceleration state so as to reduce a vehicle resonance accompanying an operation of the engine. The vehicle resonance reducing section estimates a torque to be required for the engine on the basis of an accel-operating amount due to the vehicle drive and the engine operating state and corrects the estimated torque by using operation variables of the motor vehicle so as to calculate a target throttle opening degree. Also included in the control apparatus is a lock-up control section for compulsorily locking up the automatic transmission when the vehicle resonance reducing section is in operation. That is, the lock-up control is executed when controlling the opening degree of the throttle valve so as to reduce the vehicle resonance. With this arrangement, it is possible to improve the driveability of the motor vehicle by reducing the vehicle resonance concurrently with enlarging the lock-up region to improve the fuel consumption.