In a two stroke fuel injection type internal combustion engine provided with intake and exhaust valves, a basic amount of fuel to be injected is calculated in accordance with an engine load and engine speed. Values of a factor corresponding to a ratio of air captured by an engine cylinder for combustion to the total amount of air introduced into the engine are stored in a memory in accordance with combinations of an engine load and engine speed. A value of a captured air factor corresponding to a detected combination of an engine load and speed is calculated by a map interpolation. The basic amount is corrected by the captured air factor so that a desired air-fuel ratio is obtained. The captured air factor may be further corrected in accordance with a secondary engine condition, other than an engine load and speed, such as a cooling water temperature, intake air temperature, atmospheric air temperature, atmospheric air pressure, exhaust gas temperature, or intake pressure.