On the assumption that the oil comes up by the action of "a non-return motion of the oil", the amount of oil which comes up through the clearances about the oil control ring and the top land of the piston was calculated on the basis of the hydrodynamical theory. A Nissan's automotive engine (1 200 cc, 4 cylinder water cooling) was run on the bench, and the oil loss when these clearances were changed was measured by a special precision device. As a result, the following fact was obtained, i. e. the clearance between the cylinder wall and the top land of the piston, and the perfectness of the contact of the oil control ring with the upper side of the groove, and also with cylinder wall extremely affect the oil loss.