PURPOSE: To enable to machine various kinds of screw threads by only a variation of a setting value, by providing a control system making feedback control of a motor based on the value set up for both a tap pitch setting circuit and a main spindle shifting guantity setting circuit.

CONSTITUTION: A tap 15 is fixed to a tap holder 14 at the tip of a main spindle 5 and the main spindle 5 is rotated in a positive and negative directions by a reversible motor 1. Reciprocation in the vertical direction of the main spindle 5 is performed through an ascending and descending motor 12 and gears 10, 9 and 7. An output side of a tap pitch setting circuit 18 for setting a rotary speed of the motor 1 is connected with an input side of a main control circuit 20. An output side of a main spindle shifting quantity setting circuit 19 for setting a rotary speed of the motor 12 is connected with an input side of the main control circuit 20. Measured values of gyrating mass of the motors 1 and 12 is compared with values set up for setting circuits 18 and 19 before hand and a titled control is performed by signaling so that a deflection becomes zero.