PURPOSE: To remove machining chips speedily and prevent abnormal electric discharge completely by making an electrode retract in the direction away from machining face of a workpiece and in the direction opposite to the advance direction when retraction becomes necessary.

CONSTITUTION: When retraction becomes necessary, an electrode 1 is not returned in a path L1 from which it comes, but is retracted in the direction away from machining face of a workpiece and in the direction opposite to the advance direction. That is, it is moved along a path L2 by offset amount H1. Consequently, abnormal electric discharge does not occur between workpiece and electrode, and machining chips are removed effectively.