A cylinder air filling amount is divided into a first amount of air and a second amount of air, the first amount of air and the second amount of air are calculated, and the first amount of air and the second amount of air are totaled to calculate a cylinder air filling amount. The first amount of air is the excess of the cylinder air filling amount with respect to the throttle valve air passage amount occurring due to an intake stroke being performed. The drop in intake pressure occurring due to an intake stroke being performed is detected for each cylinder and the total value of the intake pressure drop in a 720.degree. crank angle range is calculated. The first amount of air is calculated based on an intake pressure drop and the intake pressure drop total value. Due to this, it is possible to simply and accurately calculate a cylinder air filling amount.