PURPOSE: To provide stable and effective processing by calculating the dispersion value of the passed time from voltage impression to electric discharging by a dispersion value calculator, judging thereupon whether the processing is normal, changing over the electrode feed controlling condition automatically, and thereby avoiding abnormal processing.

CONSTITUTION: In accordance with no-load time of each discharge waveform measured by a Td measuring apparatus, a discharge waveform classifier 13 classifies the discharge pulses into immediate discharge D1, normal discharge D2, and open D3, which are counted by respective counters 14W16, and with this frequence a controller 18 performs electrode servo feeding. On the other hand, the dispersion value of the no-load time Td is calculated by a Td dispersion value calculator 26 with the aid of a memory 25, and a processing condition judging device 27 judges whether the processing condition is normal or not depending upon the calculation exceeding the specified level, and if abnormal, a control device 28 performs automatic changeover of the electrode feed control condition (servo reference voltage). Thus abnormal processing can be avoided certainly, and stable and effective processing be made.