PURPOSE: To work easily a working groove having the groove width larger than the diameter of a wire electrode in a short time by inclining a wire guide for guiding the travelling wire electrode along a guide groove at a predetermined angle to the moving direction of wire guide.

CONSTITUTION: When a working groove 18 having the groove width W1 larger than the diameter d of a wire electrode 5 is obtained, a wire guide 4 is pivoted at an angle θ to the axis X about the axis C of a spindle 1 to be inclined at the angle θ to the progressive direction. Then, the projected area of wire electrode 5 acting on a workpiece 17 is increased widthwise and the wire guide 4 is shifted under such condition so that the workpiece 17 is electric discharge machined to have a working groove 18 having the groove width W1 larger than the diameter d of wire electrode 5. Further, is the wire guide 4 is not inclined to be moved vertically, the working groove 18 will have the groove widthW as same as the diameter d of wire electrode 5. Thus, said working groove having the groove width larger than the diameter of wire electrode can be easily electric discharge machined in a short time with few working manhours.