A power supply control unit of an electric discharge machine comprising a means (high-pass filter (4), rectifier (5) and an integration circuit (9)) for detecting the high frequency components of a voltage in the machining gap, a detector (40) for detecting the machining voltage level in the machining gap, a high frequency component comparator (78) for comparing the detected high frequency component with a reference value, a voltage level comparator (41) for comparing the detected voltage level with a reference value, and a pulse controller (43) for controlling the idle time based on the comparison results from the high frequency component comparator and interrupting the discharge pulse based on the comparison results from the voltage level comparator.