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It has been considered that a backfire occurs due to such a phenomenon that the ignition at the overlap top dead center or the reversing of the hot burnt mixture during the valve-overlapping period triggers ignition. We have recently determined that there is a different type of ignition source that cannot be explained by the conventional theories. That is, there is an extremely slow burning cycle that continuously burns even at the exhaust stroke. That extremely slow burning lasts even after the exhaust stroke and ignites the fresh mixture during the following intake stroke, causing a backfire. In this study, we clarified procedures of the extremely slow burning that serves as a source of ignition to trigger a backfire by the pressure measurements at the intake and exhaust ports and visualizing the phenomena with the high-speed CCD camera. In this article we report the outlines along with the considerations on the factors that affect the extremely slow burning cycle.