This paper describes an experimental study on the ignition of unsteady hydrogen jets injected into compressed high temperature air behind reflected shock waves. Hydrogen gas was injected into the test section 8.5 -11 ms before incident shock wave were reflected from the end plate. Pressure measurements as well as the detection of light emission due to combustion of a hydrogen-air gas mixture were made with use of a photomultiplier and pressure gages. The results are : (1) The ignition delay time observed under the temperatures ranging 800 to 1600 K were from 0.5 to 15ms. (2) There exist ignition limits similar to the experiments with premixed gases. However the authors did not recognize clearly "explosion-peninsula". (3) The ignition delay time of unsteady hydrogen jets injected into the shock compressed air was longer than that of the hydrogen-air mixture, because finite duration is needed for mixing. (4) The low temperature hydrogen spurted into the high temperature air acts to decrease the temperature in the test section. As a result the ignition delay time for the unsteady hydrogen jet becomes longer.