To stabilize calculation accuracy of an intake air amount of an internal combustion engine 10, regardless of a state of a tumble generation valve (TGV44) disposed downstream of an intake pressure sensor 42 and upstream of a combustion chamber 24.

About a relation among intake pressure detected by an intake pressure sensor 42, a revolution speed detected by a crank angle sensor 22, and the intake air amount, two sets of map data in a full-open state of the TGV44 and in a full-closed state of the TGV 44 are prepared. According to opening , weighted average processing of the intake air amount mapped and calculated by the two sets of map data is performed to calculate a final intake air amount.