Some experimental studies are made on the oil film thickness of the piston-ring which was obtained theoretically in the 1 st Report. The same apparatus as in the 2 nd Report is used, and the 1 st method of this work is a measurement of the electric resistance between the cylinder and whole body of the piston-ring or its certain local parts, and the 2 nd method is a measurement of the circumferential displacement of the gap of ring. From these experiments, it is shown that the appearance of variation and the absolute value of the thickness of oil film measured against the phase of a stroke and a μU/W which is determined by the running condition coincide approximately with the result of theoretical calculation.