PURPOSE: To prevent redisconnection from occurring after repairs, by controlling a discharge energy momentum in time of machining resumption from a disconnection spot of a wire electrode after repairing the snapped wire so as to make it smaller than that of energy in time of the disconnection.

CONSTITUTION: In the case where a normal work 1 is machined, all relays 28, 29 and 30 are in a conductive state and therefore pulsative discharge energy is fed to the work 1 and a wire electrode 12 by means of on-off operations in transistors 22, 23 and 24. Repairing the snapped wire, when putting back to a disconnection spot and reopening the machining, a control circuit 18 of a numerical control system 17 outputs a signal, opening the relay 28, and a resistor, where an electric current to be turned to the discharge energy passes, is set to only relays 26 and 27, thereby reducing the electric current. With this constitution, the discharge energy is made smaller than that before the disconnection so that redisconnection of the wire after being repaired is preventable.