Cleaner and more efficient combustion engines have been expected to suppress the consumption of petroleum resources and to save the environment of the earth. From such a standpoint, lean mixture has been thought to be suitable. Recently, the cars equipping engines that can burn at equivalence ratio of approximately 0.6 have been developed. The mixture whose strength is lower than 0.6 is thought not to be appropriate for internal combustion engines because such engine cannot afford stable driving. So, the mixture whose strength is lower than 0.6 must be augmented by any new devices. Several jet ignition methods such as plasma jet igniter have been proposed. Most of them are thought that the principal factor of augmentation is radicals. Strong jet is also a factor of augmentation. Authors had the latter standpoint and proposed hydrogen jet igniter. It uses small amount of hydrogen. The combustion processes are similar to that ignited by plasma jet. It is very important to make clear the mechanism of augmentation by such jet igniters to adapt them to the actual engines. In this report, the fundamental characteristics of the hydrogen jet igniter are briefly mentioned and the mechanism of augmentation is investigated. The role of radicals is separated from the role of flame jet speeds.