Abstract: A method of control of rotation of a spindle device and a control system of a machine tool which enable machining without generation of chatter vibration without using special tools of variable pitch cutting edges or chatter vibration detecting means. The method of control of rotation of a spindle device and control system of a machine tool according to the present invention store spindle rotational speed change data, which determines how to change a rotational speed of a spindle in accordance with an instructed speed, linked with machining conditions and stability limit data, select, from an instructed spindle rotational speed and machining conditions and the stored stability limit data, spindle rotational speed change data giving less vibration, and use the selected spindle rotational speed change data as the basis to change the rotational speed of the spindle.