Gas engine-driven heat pump air-conditioners (GHP) have compressors powered by gas engine. We are developing a pre-chamber spark plug to improve the thermal efficiency of GHP. The pre-chamber spark plug is a spark plug that has a cup with some holes, and it can add an auxiliary combustion chamber to a single combustion chamber engine. We found that the thermal efficiency of engine at partial load was improved by using the pre-chamber spark plug instead of a standard spark plug. It will be able to effectively improve GHP efficiency much higher.