To change machining conditions during screw cutting by calculating a servo follow-up error change quantity based on the differential rotating speed between a main spindle rotating speed program instruction value and a main spindle rotating speed instruction value and a screw pitch instruction value, controlling a main spindle angle, and controlling a screw cutting start.

Each block of a machining program in a machining program memory 1 is read out in sequence by a program interpretation section 2, and function generation is executed by a function generation section 3. When the machining program includes a screw cutting instruction, a main spindle rotating speed program instruction value D1 is sent to a main spindle rotating speed instruction comparison section 5 and a main spindle override calculation section 4 from the program interpretation section 2. The instruction value D1 is multiplied by main spindle override to calculate a main spindle rotating speed instruction value D2, and both instruction values D1, D2 are compared with each other. A servo follow-up error change quantity is obtained from a differential rotating speed D3, a screw pitch instruction value D4 and the position loop gain of a servo system, and it is converted into a main spindle angle by a screw cutting controller 7.