The authors have proposed a detection method of the decomposed gas of SF6 in gas insulated switchgear (GIS) using carbon nanotube (CNT) gas sensor. The previous studies focused on the decomposed gas generated by the partial discharge (PD) under AC or DC high voltages. In this paper, we demonstrated an application of CNT gas sensor for a fault localization method in case a grounding fault was occurred in GIS. The CNT gas sensor responses to SF6 decomposition gases generated by an impulse arc discharge were measured for both impulse voltage polarities. Chemical gas analysis was also conducted to identify decomposition products, which were responsible for the CNT sensor response.