Cogeneration (also combined heat and power, CHP) systems have become widely used in recent years for reasons such as efficient use of energy resources and CO₂ emissions reduction. As a power source in cogeneration, gas engines are widely used. Large gas engines have a different ignition system from those in medium-sized and small ones. To burn the fuel mixture to the cylinder wall, many large gas engines (2MW or more outputs) with wide cylinder-bore have a pre-chamber. Flame jets from the pre-chamber enable rapid combustion of super lean mixtures throughout the cylinder walls, achieving improved heat efficiency. In contrast, many medium-sized to small gas engines (less than 2 MW outputs) have a general and simple open-chamber, in which a flame kernel produced by the electric spark of a spark plug ignites the mixture in the combustion chamber. Accordingly, open-chamber gas engines are inferior to pre-chamber gas engines, specifically in rapid and lean combustion conditions that are intended for thermal efficiency improvements.This paper is a report on a pre-chamber spark plug (hereinafter "PC plug") developed for open-chamber engines. With a pre-chamber added to a spark plug, the PC plug achieves rapid and lean combustion as in a pre-chamber engine. Meanwhile, combustion characteristics and other features have not yet been determined regarding the use of the PC plugs. In the present study, we therefore empirically assessed combustion characteristics dependent on the PC spark plug. This includes plug design and its benefits, as well as effects of PC plug design factors on combustion. The result is successful development of a PC plug with remarkably improved performance relative to standard plugs.