In the present work, self-ignition of high-pressure hydrogen released to atmospheric air through a diaphragm has been visualized under various test conditions. The experimental results show that a cylindrical flame is generated in the test tube after the self-ignition and that it continues to travel to the ambient. The present results suggest that the hydrogen which jets through the rupturing diaphragm is mixed with the heated air near the tube wall. The self-ignition event originates from this mixing, which is strongly dependent on strength of the incident shock wave generated at the diaphragm rupture. As a result, the position of the self-ignition shifts to downstream as the rupture pressure decreases. The cylindrical flame tends to become longer as it propagates in the downstream direction. Moreover, modified self-ignition mechanism is proposed based on the experimental observation.