This paper presents a finishing method of a cylinder based on the three-point measuring method of roundness. The principle of the finishing method is theoretically considered. In the three-point measuring method with a V-block, a value of f f [=1+cosn (90°+θ/2) /sin (θ/2)] means a magnifying power of measurements, where θ is the angle of the V-block and ii is a number of the polygonal cross section of the cylinder. The value of f can also determine whether the roundness of the cylinder can be measured or not, because in the only case of f≠0 the roundness can be measured. A cylindrical specimen is finished by a double threepoint method with two angles (θ=60° and 180°) having a value of f≠0 for each of n (=2, 3 and 4). A final out-of-roundness was less than 0.2 μm by the new double three-point lapping machine. Merits of this finishing method are as follows : 1) The finishing procedure is simple and inexpensive. 2). The out-of-roundness of a specimen finished by the new lapping machine is not influenced by the accuracy of the lapping machine itself.