To detect the amount of intake air approximate to a true value at all times by updating learned values for the amount of intake air calculated using a D-Einspritz system averagely in all operating regions.

When the amount of intake air GAIR is calculated from a primary equation GAIR=α×Pm+β in accordance with intake pipe pressure Pm, coefficients α, β are found from α=αo+α(k), βe=βo+β(k). The learned values α(k), β(k) are learned when an air/fiel ratio feedback coefficient λ sticks to an upper/lower limiter λmax or λmin for a set time and an engine is in a steady state. If λ≥λmax, they are updated to a value that a set corrected value αO or α1 is subtracted from the learned value α(k) and to be a value that a set corrected value βO or β1 is added to the learned value β(k), respectively. If λ≤λmax, they are updated to be a value that the set corrected value αO or α1 is added to the learned value α(k) and to be a value that the set corrected value βO or β1 is subtracted from the learned value β(k), respectively.