Knocking phenomenon in a spark ignition engine breaks out due to autoignition in the unburned gas region. Investigation on the pre-autoignition reaction, that is, the reaction of cool and blue flames happening before autoignition must be carried out in detail to control knocking. The reactions appear in an extremely short time before autoignition, so, much difficulties accompany an attempt to grasp the situation.In the experiments presented hear, progress situation of pre-autoignition reaction was made clear by visualized phenomena in a rapid compression and expansion machine (R.C.E.M), which had good reproducibility. Taken by two ultra high-speed video cameras. We determined the ignition delay time was caught by analyzing the emission of light from the combustion chamber before knocking occurrence. The catching system composed by interference filters detecting the luminescent wavelengths cool, blue, and heat flames, after branching by a 4-branched optical fiber and photo multiplier tubes. And, characteristics of pre-flame reaction was analyzed by using the ignition delay time and calculated unburned gas temperature.The following conclusions are drawn from these experiments. The pre-autoignition reactions occur in the end-gas region, and move toward the propagating flame from the end gas. The clear rise of pressure in the combustion chamber is not recognized at this time. This reaction is caused by the blue flame reaction. The heat flame due to autoignition occurs continuously, and it is outbreak almost same place and the course in the case of pre-autoignition reaction. The pressure rises rapidly after outbreak of this heat flame, and the knocking phenomenon is accompanying with the pressure vibration. The time lag until generation of the heat flame shortens with rise of unburned gas temperature. Namely, this reaction belongs to region where the blue flame is dominant.