Plastics are now widely used in every field of industry. The use of plastics with superior characteristics has increased in such fields as precision machinery, electronics, and optics. Plastic parts in these fields must be produced by machining methods, instead of molding processes, because of the need for high accuracy, superior surface smoothness, and lower cost. The soft metals used for making laser equipment are machined by a single crystal diamond tool with a highly precise rigid cutting machine. The ultraprecision diamond turning of plastics using a high precision air-bearing spindle has been pursued at Saitama University. Recent results are described in this paper.