In this study, in order to clarify the mechanism of preignition occurrence in highly boosted SI engine at low speed and high load operating conditions, directphotography of preignition events and light induced fluorescence imaging of lubricant oil droplets during preignition cycles were applied. An endoscope was attached to the cylinder head of the modified production engine. Preigntion events were captured using high-speed video camera through the endoscope. As a result, several types of preignition sources could be found. Preignition caused by glowing particles and deposit fragments could be observed by directphotography. Luminous flame was observed around the piston crevice area during the exhaust stroke of preignition cycles. This implies that the lubricant oil or mixture of oil and liquid fuel which is accumulated in the piston crevice area burns under low oxygen condition, and that the glowing particles which induce preignition would be produced in the oil combustion during the expansion stroke. Preignition induced by a lubricant oil droplet ignition was probably captured using fluorescence technique. Through the investigations, preignition which occurs at the piston crevice area was often observed. It was also found that preignition event often terminated in a single cycle in the present study and hardly occurred in the sequential manner including normal SI combustion as has been frequently reported in the previous studies. This is probably because a scavenging efficiency in the engine operating condition in this experiment is high. In the conventional boosted gasoline engine, an exhaust pressure increases, and some of the particles generated during first preignition cycle are not scavenged and remain inside the cylinder. The residual particles are heated during the subsequent combustion cycle and induce the preignition again. Preignition event continues until the residual particles are consumed. This would be one of the possible mechanism of the sequential preignition occurrence.