An engine control system is a system configured to selectively switch between first fuel and second fuel and perform an operation control of a single engine, including: a sensor configured to detect an oxygen concentration of an exhaust gas that is exhausted from the engine; and a control device configured to perform an air-fuel ratio feedback control such that an air-fuel ratio becomes a target air-fuel ratio based on an output signal of the sensor, wherein the control device calculates a correction coefficient of an air-fuel ratio feedback control during an operation with the second fuel, and further stores a fuel composition correction coefficient that is a value in a predetermined range and corrects a difference between the calculated correction coefficient and a targeted correction coefficient arising from a change in composition of the second fuel.