A machine tool includes a rotary table on which a workpiece is mounted, a clamping mechanism for clamping the rotary table against rotation, and a sensor for detecting completion of a clamping operation of the clamping mechanism. A numerical controller for controlling the machine tool calculates the time required for actual operations of the clamping mechanism, taking into account beforehand the processing time for processing a feedback signal from a sensor and the time until the next operation of the machine tool. By controlling the clamping mechanism based on the calculated required time, the cycle time of clamping or unclamping operations is shortened.