PURPOSE: To certainly and smoothly form dynamic balance by installing a supersonic-wave vibrating tool onto a working apparatus and carrying-out working with the dynamic balance or the turned resonance through noncontact work or light contact work, observing the vibration state.

CONSTITUTION: A supersonic-wave vibrator 1a performs supersonic-wave vibration, receiving an excitation voltage having a turned resonance frequency from a supersonic-wave electric power source 5, in the state having a vibrating tool 1, and installed the electric voltage pulse in the desired electric-discharge condition is given from a working electric power source 4 into between a working electrode 2 into which servocontrol feed is given and the vibrating tool 1c, and the electric discharge machining by the vibration tool 1c is carried-out. While, the frequency of the voltage pulse of the working electric power source 4 is set nearly equal to or higher by 10W30% in comparison with the turned resonance frequency, and the dynamic blalance of the apparatus 1 is formed by allowing the frequency of the generated electric discharge to nearly accord with the excitation vibration frequency supplied from the supersonic-wave electric power source 5, and the accordance of frequency is detected by a detector 6, and working is suspended.