It is known that a cylinder-bore shape affects engine oil consumption. This study aimed clarifying the conformability of an oil-ring under engine operating condition. Oil film thickness at the sliding surface of oil-ring rails was measured by laser induced fluorescence (LIF) method using optical fiber embedded in the oil-ring. Findings showed that the piston tilting motion affected oil film thickness. Furthermore, thicker oil film was found at the following rail than that at former rail. It suggested that oil was supplied to the following rail from not only the former ring sliding surface but also somewhere, for example, the oil ring groove.