Jet ignition was recognized in the 1970s, and has since been applied to automobile engines such as the Honda CVCC. In the 1990s, jet ignition was observed in explosions and was seen as a problem that may relate to jet ignition. Our group presented jet ignition experimentally and numerically in 1999, and later using LIF measurements with the same experimental vessel as used in 1999. However, the detailed mechanism of jet ignition was not clarified at that time. The target of this study is to clarify how jet ignition happens and to understand the detailed mechanism of flame jet ignition.