A spark plug with flanges attached to a conventional spark plug has been proposed as a method for reliable ignition in a lean mixture with strong turbulence. When the spark plug is applied to a cogeneration engine, the maintenance cost is expected to be reduced by the reduction of the replacement frequency of the spark plug. Ignition probability tests in mixture were conducted using conventional and newly devised dome flanged spark plugs. The shock energy recovery effect of the dome-shaped flange confirmed that that reliable ignition can be performed with small ignition energy. Furthermore, as a result of the durability test in the high temperature and high pressure atmosphere, it was confirmed that the consumption of the electrode can be suppressed by reducing the spark energy. A spark plug having a concentric ground electrode has a disadvantage that the ignition probability is low even when large spark energy is given due to the small spark gap. However, it was clarified that the ignition characteristics are improved even with small energy and the durability is also improved by the reduction of the ignition energy by the increase of the spark gap and the installation of the dome-shaped flange.