An engine control system for diminishing driveline vibrations extracts resonant frequency component coincident in frequency with a driveline resonant vibration according to a gear ratio of a transmission of the driveline from a change in charging efficiency with time that is detected on the basis of a signal representative of an amount of intake air, converts the resonant frequency component to a demanded amount of engine torque reduction after having adjusted the resonant frequency component in phase, and causes the demanded amount of engine torque reduction by retarding an ignition timing and/or reducing an amount of fuel injection.