Abstract: A method for boring small holes in a substrate by disposing electrodes on both sides of a substrate to be machined and applying a voltage between the electrodes so as to perform plasma discharge treatment at a pressure of 10 to 2.times.10.sup.3 Torrs. In another method, the gap of the two electrodes is set to 0.01 to 50 mm and a peak voltage to 10 V to 50,000 V in accordance with the electrode gap within the range of .tau..sub.ON the voltage waveform of 5 .mu.s to 20 seconds. The methods make it possible to quickly and reliably accomplish removal of chips causing clogging, smoothing of the inner walls of the holes and removal of fins in the holes.