Plasmas produced during $CO_2$ laser welding is spectroscopically analyzed. The irradiation of $CO_2$ laser beam on a sample generates plasma from the metal vapor and ambient gases, the generated plasma enhances the absorption of the incident laser beam; however, it can prevent the penetration of the absorbed laser energy to the sample. Three kinds of shielding gases, He, Ar, and $N_2$, were blown on the welding spot to remove partly the plasma generated during welding. A 4 kW CW $CO_2$ laser is focused on S45C carbon steel with off-axis parabolic mirror. The minimum focal spot size was 0.038 cm at -1 mm from the sample surface, and the sample moving velocity was 40cm/min. Time-integrated spectral data taken on photographic films showed that the identified spectral lines in the wavelength range of 3000-6500Å were mostly Fe I and Ar II lines. The time-resolved spectral data obtained with PM tube showed that there exists a significant difference depending on shielding gas used, which can be utilized for the in situ analysis of the welding condition. The electron temperature of plasma was obtained by comparing the spectral intensities of different transitions of Fe I. In the case of He shielding gas, a small, hottest plasma was generated; while, a large, cooler plasma was generated in the case of Ar shielding gas. In the case of $N_2$ shielding gas, a medium size of plasma was generated, and the temporal behavior and the temperature of plasma were similar to that of He gas.