Every time the cumulative number of revolutions of a spindle rotating motor reaches a cumulative number of revolutions set in a data table, a CPU reads out, from a data memory section, moving positions of a workpiece and tool at the reached cumulative number of revolutions and the next cumulative number of revolutions, and a spindle rotational speed instruction value at the reached cumulative number of revolutions. The CPU sets the reached cumulative number of revolutions as a start point and the next cumulative number of revolutions as an end point, and divides the interval between the start and end points at predetermined timings. The CPU determines moving positions of the workpiece and tool at divided timings on the basis of the readout moving positions of the workpiece and tool. The CPU outputs the spindle rotational speed instruction value as a spindle rotational speed instruction signal to a reference spindle rotational speed setting circuit to control the rotational speed of the spindle rotating motor.