As friction reduction of an engine sometimes causes an increase in lubricating oil consumption, the requirement of reducing oil consumption has become crucial recently. The oil used for lubrication around a piston travels into the combustion chamber via piston ring’ gap, side and back, and sliding surface. This study focused on the oil flow via the side of the lower rail of the three-piece type oil control ring (OCR). Oil film thickness was measured by the laser induced fluorescence method. Oil film thickness between the lower rail and the lower flank of the ring groove at the outer and the inner side was successfully measured. The waveform of the oil film thickness was compared to the wave forms of friction force, inertial force, pressure under the OCR and pressure at the third land. The waveform of oil film thickness was well explained by those forces. Furthermore, it was also found that the shape of the lower rail remarkably affected the oil film thickness.