When fuel supply is stopped while an engine is decelerating, pressure in each cylinder is detected using a pressure sensor, and the peak value of the pressure in each cylinder is determined. Also, an intake air amount is detected when the fuel supply is stopped, and the allowable range of the peak value is calculated based on the intake air amount. When the peak value of the pressure in a cylinder is in the allowable range, it is determined that a charged air amount in the cylinder is in a target amount range. When the peak value of the pressure in a cylinder is out of the allowable range, it is determined that the charged air amount in the cylinder is out of the target amount range, and a warning device is operated.