Diesel spray combustion was observed in an "overtailored" shock tube. A constant-temperature gas phase was established for a sufficient length of time for spray combustion studies for Mach numbers less than 2.3. It was confirmed that this shock-tube technique is a useful tool for studying diesel spray combustion. First, using the overtailored shock tube, diesel spray combustion was studied in burnt hot gas, which was produced by combustion of an air-rich propane/air mixture. The results were as follows: (1) above [Air]/[C3H8] = 100, the ignition delay, the combustion time, and the total thermal radiation per injection were almost of the same order as those of shock-heated air. The heat of radiation corresponded to 5-9% of the combustion heat of light oil. (2) Below [Air]/ [C3H8] = 100, the ignition delay decreased from around 5 ms to less than 1 ms with decreasing [Air]/[C3H8], whereas the combustion duration increased from around 10 to 30 ms with decreasing [Air]/[C3H8]. With [Air]/[C(3)H8] = 20, the flame remained throughout the measuring time (= 25 ms), because the spray did not burn out in the oxygen-lean mixture. Second, the processes of ignition and combustion were also observed in oxygen-enriched air at constant temperature. The 50% oxygen mixture gave the strongest thermal radiation from the soot at the same initial temperature.