To perform a precisely filling of intake air by determining a reference air flow rate connected to an intake port opening and/or an exhaust port opening depending on an engine variable, determining the intake port opening generated so as to be different from a known intake port opening, and correcting the air flow rate.

A map 410 in which the air flow rate sucked at a fixed intake port opening and an exhaust port opening in a reference state is stored is used to read the air flow rate based on an engine rotating speed 400 and the actual stroke 402 of an intake valve. The maximum suction area generated from a stroke change is read from a map 20, and the valve opening area just before the top dead center is read from a map 30 depending on the opening and valve stroke of the intake valve. Further, the cylinder capacity in suction after the top dead center is read from a map 60 depending on the crank angle. The ratio of general opening to the actual opening is determined as the correction coefficient of air flow rate (means 200), and the air flow rate is corrected with this correction coefficient and outputted.