A controller for controlling a synchronized operation of spindle and feed axes. A spindle-axis control section includes an initial-motion control section for making a spindle axis perform an accelerated rotation at maximum capacity from a process start position; a maximum-acceleration detecting section for detecting a maximum acceleration of the spindle axis; a residual rotation-amount detecting section for detecting a residual rotation amount of the spindle axis; a current-speed detecting section for detecting a current speed of the spindle axis; a decelerating-motion control section for making the spindle axis perform a decelerated rotation at a gradually increasing deceleration so as to reach an intermediate rotation speed after the accelerated rotation; and a positioning-motion control section for making the spindle axis perform a decelerated rotation at maximum capacity so as to reach the target thread depth after reaching the intermediate rotation speed.

A control device calculates a specified quantile in stored data relating to a load applied on a spindle, as a target spindle load, and controls a feed speed of the spindle as to a work, so that the measured load applied on the spindle is the calculated target spindle load. A user can set the target spindle load with the load on the spindle measured in a state in which the work is being stably machined as a reference.