Engine oil consumption must be reduced for lowering particulate matter, deterioration of engine after treatment devices and users running cost. A lot of factors affect engine oil consumption, and it is usually estimated experimentally on very latter stage of engine development. Therefore calculation method for oil consumption which can be used for engine design is required. Supply oil volume is necessary to calculate oil consumption. In this study, oil pressure distribution under the oil ring which affects supply oil volume was measured, and a hypothesis for generating oil pressure was discussed. Oil pressure was deviated from crank case pressure and a pressure rise under the oil ring was found in the latter half of the piston down strokes. The maximum pressure was measured at the center of the piston skirt under the oil ring. It was showed that oil pressure rise could be simulated considering distribution of oil film thickness on the cylinder wall.