A replacement determination method: detects the remaining length of the electrode; detects the electrical discharge commencement position, being the position of the electrode when electrical discharge starts; detects the throughole position, being the position of the electrode when the workpiece is pierced; sets the required length for the electrode as required for machining the next throughhole, on the basis of the difference between the detected electrical discharge commencement position and the detected throughole position; compares the detected remaining length and the set required length; and determines whether or not electrode replacement is required.