PURPOSE: To efficiently remove smear generated in a through-hole bored in a substrate by making the X-axis-Y-axis moving speed of a substrate and/or the scanning speed of an electrode higher at a part where there is no through- hole, and making the same lower or once stopping at a part in the vicinity of a through-hole.

CONSTITUTION: A needle-like electrode scans the surface of a substrate 3 having through-holes l of various sizes while the reciprocating motion is shifted at a fixed gap. The electrode is moved at a higher speed at a part where there is no through-hole, (a), and according to the positional data on the through-hole 1 recorded in a first process, the speed is decreased as it approaches the through-hole l. When it further approaches the through-hole l, the low-speed movement is continued while discharge is started (b). When it passes the through-hole 1, the discharge is ended (c), and the speed is again increased and scanning is performed (d). Thus, in addition to scanning of the electrode, the substrate 3 may be moved in the X-axis-Y-axis direction.