It is well known that some inorganic powders show remarkable activities to extinguish fire. The authors proposed a kind of procedures to appreciate the activity and also to approach to the extinguishing mechanism. Propagating speed of flame was measured in the cylindrical combustion tube by the oscilloscopic patterns from the phototransistors attached on the wall (Fig. 1). The powder was poured from the funnel to be attached on the wall. Mixtures of methane and air of various pressures and components were used inflamable gases. The powder was (NH₄)₂HPO₄ with average surface of 2.72m²/g. The examples of results were shown in Fig. 5 and 6, and Table 2. The extinguishing activity was estimated quantitatively, and its mechanism was concluded to be due to catalytic activity of surface and also partly to thermal decomposition of powder material.