To promote discharge of sludge, prevent concentrated discharge and secure surface roughness of a side surface by providing an oscillating gap control means performing a gap control between an electrode and a work piece, separately from a revolving motion or a radiating motion within an oscillating surface.

An oscillation control part 34 calculates moving amount Δx and Δy per unit time for every axis for performing constant speed oscillation, regardless of a gap control by an oscillation shape and oscillation speed, etc., transmitted from an NC program interpretation part 32 and the present location transmitted from an interpolation processing part 33 and transmits the calculated moving amount Δx and Δy to a servo control part 38. The servo control part 38 controls revolution of each axis motor and enforces the operation of each axis, based on the vector composition result of moving amount Δx, Δy and Δz per unit time of the side line direction of a truncated cone which is transmitted from the interpolation processing part 33 and moving amount Δx and Δy per unit time of oscillation transmitted from the oscillation control part 34.