PURPOSE: To enable high-precise machining of a shape with a low difference in stage of a machining seam, by a method wherein machining is effected in a state that a tool is preheated to temperature near a tool temperature estimated during actual machining to reduce a temperature change of a tool due to machining.

CONSTITUTION: A tool T attached to a main spindle 10 by means of a twin arm is positioned concentrically to a heating hole 36a of a preheat device 25 through movement of a main spindle head, and with a main spindle quill 9 lowered, the tool is inserted in the hole 36a. After the tip of the tool T is brought into contact with the bottom of the hole 36a, a moving body 28 is pressed down. A dock tip 31a is brought into contact with a limit switch 32 to output a tool input signal from a switch 32 to an NC, and a voltage is fed to a heat generating body 37 to start preheat. The temperature of the tool heated within the 36a by means of a heat generating body 37 is detected by a measuring sensor 41 to output the result to a temperature control device, and when the temperature of the tool is increased to a set value near a saturation temperature available during machining, a heating signal is outputted to the NC. The quill 9 is raised, the tool T is pulled off from the preheat device 25, movement of the main spindle head positions a cutting restarting position, and cutting is restarted by means of a profile signal from a tracer head.