To easily correct the slant of a wire electrode to a workpiece by using the workpiece which is machined into a product and the wire electrode without a special jig. Discharge is generated by approaching a wire electrode 1 to a workpiece fixed on a table. The slant of the wire electrode and correcting amount are calculated by measuring the discharge for a prescribed period of time or a prescribed number of times, while taking the discharge position as a width or a distribution. Then, correction is performed. Therefore, it is possible to correct the slant of the wire electrode relative to the workpiece directly by using the wire electrode. Accordingly, it is possible to drastically reduce the time necessary for getting accuracy in fixing the workpiece on the table and the time necessary for arrangements such as verticality check of the wire electrode by discharge.