Ammonia is expected to be used not only as a hydrogen carrier but also as a fuel for combustion equipment toward future carbon neutral society because it is easily liquefied and has a higher energy density than liquid hydrogen. This report shows the combustion characteristics and an engine system in case of which ammonia is used as the fuel for spark ignition engines. Ammonia has low flammability, but stable combustion is possible by itself at low speed and high load operation, and it has an advantage over knocking. On the other hand, auxiliary fuel is required for stable operation under a wider range of operating conditions. By reforming a part of the supplied ammonia to hydrogen and using it as an auxiliary fuel, a single fuel supply system is configurated and CO2 emissions from the engine can be eliminated.