Hydrogen is expected to serve as a clean energy carrier. However, since there are serious ignition hazards associated with its use, it is necessary to collect data on safety in a range of possible accident scenarios so as to assess hazards and develop mitigation measures. When high-pressure hydrogen is suddenly released into the air, a shock wave is produced, which compresses the air and mixes it with hydrogen at the contact surface. This leads to an increase in the temperature of the hydrogen-air mixture, thereby increasing the possibility of ignition. We investigated the phenomena of ignition and flame propagation during the release of high-pressure hydrogen. When a hydrogen jet flame is produced by self-ignition, the flame is held at the pipe outlet and a hydrogen jet flame is produced. From the experiment using the measurement pipe, the presence of a flame in the pipe is confirmed; further, when the burst pressure increased, the flame may be detected at a position near the diaphragm. At the pipe outlet, the flame is not lifted and self-ignition is initiated at the outer edge of the jet. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.