The present paper deals with auto-ignition and combustion of high pressure hydrogen leaks through a tube with air into atmosphere. The experiments using a high pressure tube with hydrogen and an extension tube with air of 10 mm diameter are performed to clarify the auto-ignition and combustion behavior of high pressure hydrogen jet spouting through the extension tube. In order to improve repeatability and reliability of experiments, a plunger system with a needle is applied to control a rupture of a PET diaphragm, which separates hydrogen from air. The most important result obtained in the present study is that the minimum diaphragm burst pressure at which the auto-ignition is registered, is found to be between 1.0 and 1.2 m of the extension tube length. This tube length is longer compared to tubes used in the previous studies. Combustion and Mach disk at the extension tube exit to atmosphere are observed using a high speed camera which is set at the frame speed of 1 x 10(5) fps when the ignited hydrogen jet is spouted out the tube. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.