To reduce the number of models that are mounted on a computer and are used for estimating a cylinder inside intake air volume.

An electric control unit 80 estimates an intake valve passing gas volume mi and the cylinder inside intake air volume Mi for each cylinder by using an intake air volume estimating means comprising a throttle model M2, an intake pipe model M3, intake valve models M4a, M4b that are identical to each other, cylinder models M5a, M5b that are identical to each other, and exhaust valve models M6a, M6b that are identical to each other. The electric control unit estimates each intake valve passing gas volume mi or the like of the first and fourth cylinders whose intake valve opening periods of time do not overlap each other by using the intake valve model M4a, the cylinder model M5a and the exhaust valve model M6a. Further, the electric control unit estimates each intake valve passing gas volume mi or the like of the third and second cylinders whose intake valve opening periods of time do not overlap each other by using the intake valve model M4b, the cylinder model M5b and the exhaust valve model M6b.