Application of the inductively coupled thermal plasma (ICTP) technique was proposed for investigating plasma-quenching efficiency of various gases including the arc-quenching medium of SF₆. The ICTP enables us to study fundamentally the effect of gas injection on thermal plasma without any impurities because it has no electrode. Seven kinds of gases including CO₂, SF₆ and environmentally benign gases (N₂, O₂, air, He and H₂) were injected into Ar ICTP. Spectroscopic observation was carried out in order to investigate a change in the excited state of Ar atoms due to addition of these gases. Radial distributions of the radiation intensity of Ar spectral lines and the temperature of Ar ICTP were estimated. It was found that 10% CO₂ addition causes a remarkable decline of the radiation intensity and temperature at any radial position, similarly to 2% SF₆ addition. Two-dimensional local thermal equilibrium modelling for Ar ICTP also revealed that CO₂ causes temperature decay more than the other gases of N₂, O₂, air, He and H₂ except SF₆.