This paper shows the feature extraction for gas discrimination from the transient response of a gas sensor using a subspace-based system identification technique. We have proposed an approximation and analysis method for the transient response using a subspace-based identification method. This is based on Prony's method and can extract time constants and gain parameters of multicomponents of a sensor output response. The estimated parameters can be used to improve gas discrimination in multivariate analysis as elements of pattern vectors. Transient responses of semiconductor-type gas sensors were evaluated using the proposed method. We show that the method can extract features connected with characteristics of measured gases from these sensor responses.