Combustion experiments were conducted with an optically accessible engine that allowed the entire bore area to be visualized for the purpose of making clear the characteristics that induce extremely rapid HCCI combustion and knocking accompanied by cylinder pressure oscillations. The HCCI combustion regime was investigated in detail by high-speed in-cylinder visualization of autoignition and combustion and emission spectroscopic measurements. The results revealed that increasing the equivalence ratio and advancing the ignition timing caused the maximum pressure rise rate and knocking intensity to increase. In moderate HCCI combustion, the autoignited flame was initially dispersed temporally and spatially in the cylinder and then gradually spread throughout the entire cylinder. In contrast to that behavior, in extremely rapid HCCI combustion, the autoignited flame was dispersed in the cylinder in its initial stage, but the remaining unburned end gas rapidly autoignited at a certain point. That gave rise to knocking combustion accompanied by cylinder pressure oscillations.