Engines using natural gas as their main fuel are attracting attention for their environmental protection and energy-saving potential. There is demand for improvement in the thermal efficiency of engines as an energy-saving measure, and research in this area is being actively pursued on spark ignition engines and HCCI engines. In spark ignition gas engines, improving combustion under lean condition and EGR (exhaust gas recirculation) condition is an issue, and many large gas engines use a pre-chamber. The use of the pre-chamber approach allows stable combustion of lean gas mixtures at high charging pressure, and the reduction of NOx emissions. In small gas engines, engine structure prevents the installation of pre-chambers with adequate volume, and it is therefore unlikely that the full benefits of the pre-chamber approach will be derived. However, recent research on pre-chamber plugs suggests that the pre-chamber combustion approach extends the lean limit even when fuel is not supplied to the pre-chamber, and that this limit is not particularly dependent on the large volume of the pre-chamber. This study, therefore, focused on specifications such as pre-chamber orifice diameter and number of orifices, with a view to reducing NOx emissions and improving efficiency in small engines with this type of pre-chamber structure and examined the reasons why the lean limit is extended in pre-chamber systems with no fuel-supply mechanism.Even in a combustion chamber with a small diameter, it is possible to shorten the combustion period using the jets of a pre-chamber plug. Placing the sparking position in the pre-chamber close to the orifices extends the lean limit. This is likely to be due to the way residual gas in the pre-chamber and new gas mixed. Residual gas, flow, atmospheric temperature and other factors influence ignition in the pre-chamber, but it was thought likely that ambient temperature in side of the pre-chamber in particular contributed to ignition and had an effect on combustion stability under lean condition. It was therefore decided to evaluate this factor using pre-chamber plugs made of three materials with different thermal conductivity: stainless steel, steel, and aluminum. Also the temperature in the pre-chamber of the pre-chamber plug was measured. Reducing the thermal conductivity of the material of which the pre-chamber is made increases the temperature in side of the pre-chamber and stabilizes the jets, shortening early-phase combustion and extending the lean limit. Furthermore one of the lowest thermal conductivity materials which are ceramic was tried to evaluate.