PURPOSE: To keep the tension of a wire electrode constant, and thereby machine a work with stable working accuracy by providing a control means controlling the generated torque of a pretension motor in response to change in radius for drawing the wire electrode out.

CONSTITUTION: The revolution of a wire supply bobbin 9 in which a wire electrode 8 is wound is detected by a first rotation detecting means 21, and the revolution of a roller 15 or a pulley the diameter of which is already known is detected by a second rotation detecting means 25 wherein the roller or the pulley is located within a wire electrode running path between the wire supply bobbin 9 and the feed roller 15. A control means 26 operates the draw-out radius of the wire supply bobbin 9 based on a ratio of the revolution of the first rotation detecting means 21 to the revolution of the second rotation detecting means 25, and on the radius of the roller 15 or the pulley, and the control means also controls the generated torque of a pretension motor 20 in response to the draw-out radius of the wire electrode, so that the tension of the wire electrode 8 is thereby kept constant.