PURPOSE: To prevent a decrease of accuracy due to a gear clutch and save energy, by switching a D/A converted output of an S code and a normal and a reverse instruction signal at normal time with speed instruction voltage and a position feedback pulse signal at code machining time.

CONSTITUTION: A control device, instructing a normal main spindle function by a central processing unit 3 and inverting speed instruction voltage by a reversible switch 5 to the polarity of positive or negative voltage to be input to a main spindle servoamplifier 6, controls a motor 2 to a fixed speed. Here the speed of a main spindle 1 used for every rotation feed, thread cutting, etc. is input to a pulse distribution controller 9. While the device, when it is con trolled as a C shaft, transmits a pulse to a deviation counter and D/A converter part 10 to be input to the amplifier 6 and detects by an encoder 11 a position of the main spindle 1 to be fed back to the deviation counter and D/A converter part 10. Here switches (a), (b) are switched by an M code programmed for switching a Z mode, but an auxiliary function switching part 12, decoding also the normal and reverse M code of the main spindle 1, gives an instruction to the reversible switch 5 to switch it.