In an electrical discharge machine for machining a workpiece by electrical discharge occurring in a gap between an electrode and a workpiece (hereafter referred to as the electrode gap, as necessary) while changing the relative position of theelectrode and the workpiece, it has been the practice to set a reference value for accurately ascertaining the aforementioned relative position so as to obtain high geometric accuracy. Conventionally, to set such a reference value, a positioningapparatus is known in which the electrode and the workpiece are brought into direct contact with each other, and this position of contact is set as the reference.

In addition, to effect very fine machining, it has been the practice to subject a rod electrode having a diameter of several tens of microns or thereabouts as the electrode 101 to dressing forming by means of reverse discharge by using a blockelectrode and a wire electrode, and to effect fine machining by means of the formed electrode 101, but there has been a problem similar to the aforementioned first problem.

To attain this object, a positioning apparatus for an electrical discharge machine in accordance with a first aspect is characterized by comprising: a power supply for applying a pulse voltage between an electrode and a workpiece; a dischargedetecting means for detecting the occurrence of electrical discharge between the electrode and the workpiece; and a means for generating a positioning completion signal on the basis of a detection signal from the discharge detecting means.

A method of positioning an electrical discharge machine in accordance with an eighth aspect, wherein a pulse voltage is applied between an electrode and a workpiece by a power supply, and a mean voltage between the electrode and the workpiece isobtained by a mean-voltage obtaining means, so as to control a gap between the electrode and the workpiece on the basis of a mean voltage value of the mean-voltage obtaining means, is characterized by comprising the steps of: detecting the occurrence ofelectrical discharge between the electrode and the workpiece by a discharge detecting means; counting a detection signal from the discharge detecting means a predetermined number of times by a counting means and generating a counting completion signal;and short-circuiting the electrode and the workpiece at an arbitrary voltage by a short-circuiting means on the basis of the completion signal from the counting means.