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Gas circuit breakers have been developed to be small and have low operating energy. To reduce their operating energy, a self-blast technique has been used. The self-blast gas circuit breaker obtains high pressure by utilizing arc energy in a fixed-volume chamber. Consequently, the temperature of the blasting gas rises. However, when the gas temperature rises, the interrupting efficiency of SF 6 gas decreases. Therefore, gas temperature is an important parameter that determines the interrupting performance. Gas temperature and pressure in a thermal puffer chamber of a self-blast circuit breaker using a model circuit breaker were measured by using a parameter of interrupting current and arcing time to estimate the blast gas temperature for a large current. Gas temperature in the thermal puffer chamber of the model circuit breaker was measured by a micro-gap discharge method in which the gas temperature was calculated by the breakdown voltage of a micro-gap by using critical electric field strength data. Two micro-gaps were installed in a thermal puffer chamber. The measured behavior of the hot gas was compared with flow analysis. The hot gas mixing processes with the parameter of interrupting current and arc duration time were estimated. The inlet gas temperature depends on the interrupting current, but the blowing gas did not depend on the arcing time at high currents. Results show that the temperature of the gas flowing out from the thermal puffer chamber reaches 2000 K for a' large interrupting current of 57 kA.