PURPOSE: To enable machining condition to be expressed in a pattern convenient for use, by showing a normal discharge condition by a proportion, in which a number of normal discharges are generated in a predetermined time, while the other condition by a proportion in which a time of said other condition appears in a predetermined time.

CONSTITUTION: A power supply unit 19 in a machining circuit 3 charges capacitors Cd1Cn with predetermined voltage of a DC power supply 11 by actuating a base driving circuit 17, and predetermined electric discharge machining is performed by an insulation breakdown of opposed electrodes 9. And a machining condition detecting circuit 5 detects a normal discharge by a predetermined drop of interpole voltage VG, waiting for the capacitors being charged after their normal discharge and detecting a condition of the interpole voltage to be in high, intermediate and low potentials. By these detec tion signals, an information output circuit 7B shows a normal discharge condition by a proportion, in which a number of normal discharges are generated in a predeter mined time, while an abnormal condition of no discharge, contamination, short-circuit, etc. by a proportion of a time, said abnormal condition appears, to the time of the other condition in a predetermined time. In this way, a machining condition can be expressed in a pattern convenient for use.