The engines with direct hydrogen injection have the advantage of very low emissions except for nitrogen oxides (NOx), the output power theoretically 1.2 times as large as that of gasoline engines and the complete elimination of the abnormal combustion. There is, however, difficulty in injecting a large amount of hydrogen gas because of the injection in gas phase. There were two disadvantages of the previous injection system. One was that the thermal efficiency decreased with the increase of the engine speed. The injectors needed longer injection time to inject gas fuel rather than liquid one. The other was that the combustion by using the previous injection system generated NOx to a fairly large extent even in lean mixture operation. Appropriate combustion control methods have been required. In 1975, S. Furuhama et. al. (1975) studied a hydrogen fuelled engine with a combination of external mixture formation and internal one. They found that an ideal combustion would have been obtained if appropriate combination had been found. At that time, they could not optimize the combustion because of the mechanical and hydraulic injection system. To overcome the disadvantages, an injector with high response injection, a large amount of hydrogen injection at high pressure in a short time and the injection made anytime is required. A new generation hydrogen high pressure injector developed this time for a 7.8-liter, 6-cylinder hydrogen engine is a common-rail type injector based on a common-rail type diesel injector. This injector can inject a large amount of hydrogen in very short time owing to the extremely high hydraulic pressure of the common-rail system, resulting in application of higher pressure hydrogen and a swifter and larger lift of the needle valve than those of the previous injector. And the injection timing can be changed with ease thanks to the electric control system.