PURPOSE: To remove sludge etc. forcedly by sensing the time distribution condition from voltage impression till generation of electric discharge, judging whether it is normal or abnormal discharge, and then controlling the pressure of machining liquid so as to restitute to normal condition.

CONSTITUTION: Time distribution condition in the gap between electrode 10 and a work to be machined 14 from impression of voltage till generation of electric discharge is sensed to be compared with the reference distribution condition, and the result is used to judge the condition of the interpolar gap, and if the state of the gap is considered to have worsened, a sensing signal SA is emitted. In accordance with the sensing signal SA, a controller 106 opens a solenoid valve 101 to allow a pump 100 to supply the machining liquid to a jet path 104, and there the liquid pressure is boosted above the pressure liquid obtained with a manual valve 102, and if a certain specified pressure value is exceeded, a feedback signal from a liquid pressure meter relay 105 maintains the pressure at a proper set value. Thus splash etc. produced in the interpolar gap can be exhasuted effectively to ensure a well improved machining efficiency.