To automatically and continuously machine a screw boss by setting a machining process in such a manner that, when a cutter is lowered to a preset bottom dead center, it is raised and stopped, a plunge is lowered, a stopper is released, a work is released, and a processing program is returned to its initial operation and waits there.

The feed per revolution of a cutter raising/lowering motor 6 and the rotation speed of a cutter rotation motor 14 are controlled according to a prescribed processing program so that a cutter 7 operates in a specified way so as to cut or stampingly form a groove shape in the internal diameter part, of a work 3. When the cutter 7 is lowered to a preset bottom dead center, a sensor 13b issues an instruction signal so that both motors 6, 14 move inversely to each other, the cutter 7 is raised to stop at the position of a sensor 10b, a plunger 11 is lowered, a stopper is released, the work 3 is released, and the processing program is returned to its initial operation so as to wait.