To perform the remarkably precision work in addition to a high surface characteristic by intermittently performing the working, determining the contact of an electrode with a work during the interruption time by the electrical measurement to obtain the pulse impact coefficient, and determining the frequency of the contact.

The surface machining of a work is intermittently performed with the precision pulses to finish the precut work 6 with the desired surface characteristic of 0.1 μ. The percentage of the average contact of an electrode 5 with the work 6 to the measuring time is determined on the basis of the pulse impact coefficient in the measuring module 4, and the moving speed of the electrode is lowered and the machining time is increased when the pulse impact coefficient is over a reference value. Particularly the oscillation state of the electrode is recorded and stabilized through the pulse impact coefficient, and the interference self control effect of the electrode is solved with the pulse frequency of about more than 100 kHz, whereby the error in the precut can be prevented, and about 1 μm or less of the angle part error can be achieved.