To provide a manufacturing method for an optical part or its mold, including manufacturing conditions of a fly cut machine tool capable of machining the machined surface of a workpiece to a free curved surface whose surface roughness is within about 10 nm.

According to this manufacturing method, a tool is rotated round its axis with the cutting edge directed to the outside, a machining object surface of the workpiece is cut by one portion of a circle drawn by the cutting edge, and simultaneously the tool and the workpiece are relatively moved at a predetermined feed rate to perform cutting for one line. Subsequently, the tool and the workpiece are relatively moved in the pitch direction only by pick feed amount to perform cutting for the next one line. Cutting is performed under the following conditions. The rotating speed of the tool (S): 10,000 to 50,000 rotation/min, feed speed (F): 300 to 2,000 mm/min, the cutting edge rotation radius (D) of the tool: 5 to 15 mm, the pick feed amount (f): 0.01 to 0.04 mm, and the cutting edge nose diameter (r) of the tool: within 5 to 20 mm.