This paper presents a new scheme for the detection of residual carbon tetrafluoride (CF4) gas in gas-insulated switchgear (GIS) for its diagnosis. For the diagnosis of GIS, the decomposition gas species including CF4 generated by partial discharges or breakdown are analyzed. Accumulation of the gas species indicates the history of the failure of GIS. However, most of the gas species are removed by an absorbent placed in a GIS vessel. The only CF4 stays in GIS vessel for the long term. In this study, it is aimed to develop an easy and rapid CF4 detection method. Because direct detection of CF4 is difficult, an indirect detection method of CF4 is proposed, in which CF4 is processed by non-thermal plasma (NTP). In this paper, the conversion of CF4 using dielectric barrier discharge (DBD) was examined. The infrared absorption band among SF6, CF4, and CO2 was observed by Fourier transform infrared (FT-IR) spectroscopy. The amount of CO2 generated in the case of CF4 (10,000 ppm) and O2 (10,000 ppm) diluted in Ar or SF6 was analyzed. In the case of Ar dilution, CO2 gradually increased with increasing time of DBD application by using a compact CO2 NDIR sensor. In the case of SF6 dilution, CO2, CO, O2, and SO2 were detected by using a portable gas analyzer. The latter case showed a higher CO2 generation rate of 400 ppm/hr, whereas the other showed 50 ppm/hr. In addition, a zeolite-filled column was installed for removing SO2 from the processed gas.