To provide a method for detecting and quantitatively determining the deterioration with the lapse of time of an oxygen sensor.

This monitoring method for gas sensor is particularly suitable for the use in the exhaust flow from an internal combustion engine. In a certain engine operating condition, the output signal from a sensor 22 is converted into a frequency area signal by Fourier transformation. The ratio of absolute values of certain even and odd-order harmonic waves is compared with a corresponding value with respect to a new sensor. The slippage exceeding a certain limit shows a rather deteriorated sensor.