High-pressure hydrogen jet released into the air has a possibility to ignite in a tube without any ignition source. The mechanism of this ?auto-ignition? is considered that a shock wave produced after a diaphragm rupture causes the ignition by a heat up of the mixing region. In these years, flow visualization study for the auto-ignition process has been started to conduct to understand its detailed mechanism, but such ignition still has not be known well. This study deals with a observation of the auto-ignition phenomena in the rectangular tube. The results show that a flame is confirmed when the shock wave pressure reaches 1.2-1.5 MPa at the wall and that the ignition occurs near the wall, then multiple ignitions come out while the shock wave propagates and they become a flame.