This disclosure provides a method of detecting a cylinder-to-cylinder gap abnormality of an air fuel ratio with sufficient accuracy according to an individual specificity of an air fuel ratio detecting module. Particularly, the method may be used for an engine to detect the cylinder-to-cylinder gap abnormality of the air fuel ratio of supplied air fuel mixture abnormality based on an output value of a linear O2 sensor. Under a first predetermined condition, an output characteristic of the sensor is measured. The measured output characteristic is compared with a reference characteristic set in advance to calculate a correction value for correcting the sensor output according to the individual specificity of the sensor. Subsequently, under a second predetermined condition, the sensor output value is corrected based on the calculated correction value to detect the abnormality based on the corrected output value of the sensor.