A controller for controlling a synchronized operation of spindle and feed axes. A spindle-axis control section includes a section for making a spindle axis perform an accelerated rotation at maximum capacity from a process start position; sections for respectively detecting a maximum acceleration, a residual rotation amount and a current speed of the spindle axis; a section for making the spindle axis perform a decelerated rotation at maximum capacity to reach a target thread depth after the accelerated rotation; and a section for executing a torque-limiting during the accelerated rotation of the spindle axis, in such a manner that a torque command lower than a maximum torque command is given to the spindle axis at a start of acceleration, and thereafter the torque command is gradually increased so that the maximum torque command is given to the spindle axis when a predetermined time has elapsed.