A control device which controls a servo motor of a machine tool in the event of power failure is provided. The control device includes a power failure detecting unit (10′) which monitors an input power supply (1) and when power failure is detected, instructs servo motor amplifiers (3, 4) to decelerate and stop servo motors (7, 8) while maintaining a controlled state, and instructs a spindle motor (9) to suspend application of electricity, an alarm threshold changing unit (11) which, in response to a power failure detection signal from the power failure detecting unit (10′), changes alarm threshold values for direct current voltage reduction abnormality of a power circuit (2), the servo motor amplifiers (3, 4), and a spindle motor amplifier (5) from values for a normal operation to values during power failure, and a retraction operation instructing unit (12) which, in response to the power failure detection signal from the power failure detecting unit (10′), instructs servo motor amplifiers (3, 4) to perform a retraction operation by a small distance which is defined in advance.