A computing method of an engine intake air amount for determining a fuel injection amount comprises a step of computing an engine intake amount in real time by using a relational expression between an engine speed and an intake manifold pressure expressed by Q_c=k_c(-a₁N-a₂P+a₃NP+a₄N²P+a₅NP²) where Q_c is an intake air amount of an engine cylinder, k_c, a₁, a₂, a₃, a₄ and a₅ are constants, N is an engine speed and P is an intake manifold pressure.