An intake pressure is successively detected by a pressure sensor, and an
intake pressure derivative is calculated. Next, a peak pressure detecting
range for each cylinder is set based on the intake pressure derivative.
Next, an upward peak pressure and a downward peak pressure of the intake
pressure, included in the peak pressure detecting range, are detected for
each cylinder. Next, an intake pressure drop for each cylinder is
calculated from the upward peak pressure and the downward peak pressure.
The in-cylinder charged air amount is calculated based on the intake
pressure drop.