An engine control system having an oxygen density sensor arranged in an intake system at a position downstream of a throttle valve for detecting the amount of air newly introduced into the engine. The sensor is a limit electric current detection type capable of detecting a continuously changing density of oxygen. An amount of fuel to be injected is calculated in accordance with the output level of the sensor by using mapped data of the output level of the sensor signal. The sensitivity of the sensor is detected and multiplied by the output value of the sensor for obtaining a precise oxygen partial pressure. Instead of calculating the basic fuel amount from the output level of the oxygen density sensor, a normal intake pressure sensor with an intake pressure-engine speed map can be employed. In the latter case, the oxygen partial pressure from the oxygen density sensor is used for correcting the calculated basic fuel amount. The sensitivity of the sensor is also corrected.