In this study, the cause of backfire concerning an external mixture formation type hydrogen engine was clarified. It has been known that the maximum output power of the external mixture formation type hydrogen engine should be kept significantly low, because of backfire. Generally, the backfire of this type of hydrogen engine is caused by pre-ignition. In this type of hydrogen engine, pre-ignition occurred for a range of lean mixture. Under this study, therefore, the relationship between the occurrence of backfire and the temperature at the tip of the spark plug electrode, and the detection of the luminescence spectrum of the flame near the spark plug were examined and studied in relation to the spark plug ignition theory which appeared to be promising. Then the pre-ignition timing and location were studied by detecting the flame luminescence spectrum. An attempt was also made to clarify the mechanism of pre-ignition that would cause the backfire, by photographing the pre-ignition by means of a high-speed video camera. Judging from these results, it was found that the slight reaction of combustion remaining in the piston top land was the cause of pre-ignition.