In internal combustion engine, it is well-known that oil infiltrates the combustion chamber through the clearance between the piston ring and the cylinder bore with vertical reciprocating motion of the piston, leading to an increase in oil consumption. The deformation of the cylinder bore is inevitable to some extent in the actual engine because of the tightening of cylinder head bolt and heat load._As to the function of the piston ring, it is desirable that it conforms to such bore deformation. The author et al. made a glass cylinder engine in which closed piston ring gap could be visualized, based on the idea that piston ring conformability to the sliding surface of bore could be evaluated from minute changes of the piston ring gap. This newly-devised visualized engine was an in-line 4-cylinder engine, capable of running up to 6,000 rpm, in which the closed gap of piston ring could be observed minutely during engine operation. As the result of the evaluation of the piston ring conformability to bore with this equipment, it was found that the upward passage of oil was widened and the oil consumption was increased, because abnormal behavior, that is, the separation of piston ring from deformed portion of cylinder bore at the down-stroke of piston, had occurred in the area of high speed. The degree of separation of the piston ring from the cylinder bore was closely connected with the tension of the piston ring.