For continuous measurement of oil-film-thickness on the piston ring, an electrical capacitance method has been applied. The capacitance pick-up fixed in the ring had an electrode with or without an insulating surface layer. We also measured friction forces at the same time, and learned the following. (1) If oil is supplied adequately, oil-film-thickness agrees with the theoretical value. Friction force, however, was 2 or 2.5 times greater than the theoretical value. (2) Oil-film-thickness on the top ring of the actual engine is considerably smaller than the theoretical value. This is considered attributable to inadequate oil supply (starvation). (3) The piston ring sliding surface is not always parallel to the cylinder wall. This phenomenon has a major influence on the formation of partial oil-film-thickness.