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 the 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. A precise detection of new air is made possible while maintaining the advantages of the known D-J system. An ignition timing also may be calculated by the signal from the oxygen sensor.