A controller includes a determination circuit, a setting circuit, and a control circuit. The determination circuit is configured to determine whether a chatter vibration is occurring while a spindle is rotating and a workpiece is being cut. The setting circuit is configured to set an upper limit and a lower limit on a change amount of a rotational speed of the spindle. The control circuit is configured to determine the change amount randomly from a range between the upper limit and the lower limit and configured to rotate the spindle at a second rotational speed obtained by changing a first rotational speed by the change amount that has been determined if the chatter vibration is determined as occurring while the control circuit controls the rotational speed of the spindle at the first rotational speed.