Ammonia is one of the most useful compounds that react with NOx selectively on a catalyst, such as V2O5-TiO2, under oxygen containing exhaust gas. However ammonia cannot be stored because of its toxicity for the small power generator in populated areas or for the diesel vehicles. A new concept for NOx reduction in diesel engine using ammonia is introduced. This system is constructed from the hydrogen generator by fuel reformer, the ammonia synthesizer, SCR catalyst for NOx reduction and the gas injection system of reformed gas into the cylinder.Experimental results show that, the SCR catalyst provides a very high rate of NOx reduction, reformed gas injection into cylinder is very effective for particulate reduction.WHEN CONSIDERING INTERNAL COMBUSTION ENGINES of the 1990's the question of how to harmonize the engine with the natural environments is one of the greatest problems. The internal combustion engine changes a substance into energy via its explosive combustion. However, a lot of effort and energy is consumed to eliminate harmful components produced by the combustion process. If it were only possible to take out energy efficiently without producing harmful components by some catalytic conversion process. This idea is the basic concept of the “Catalytic engine” proposed here.The diesel engine, which is widely used for vehicles, has the highest thermal efficiency among the internal combustion engines and minimizes the entropy increase. The major problem to be solved in diesel engines is the elimination of NOx and particulate in the exhaust gas. If those exhaust emission problems could be overcome, the diesel engine would be further harmonized with the natural environment.