PURPOSE: To sufficiently eliminate machining chips and so on, to hold cleanliness and resistivity value of machining liquid at a satisfactory level and to prolong life of an ion exchange resin by measuring the resistivity value and turbidity or a pH value and by circulating necessary and sufficient amount of machining liquid based on the result.

CONSTITUTION: When turbidity or a pH value of machining liquid in electric discharge machining becomes more than a predetermined value, a circulation pump P1 for feeding the machining liquid to a filter F is activated. Next, the turbidity or the pH value becomes less than the predetermined value, the circulation pump P1 is stopped. Due to this, machining chips and so on can be sufficiently eliminated and life of an ion exchange resin can be prolonged with the holding cleanliness and resistivity value of the machining liquid at a satisfactory level, and thus, without affecting machining, cost and labor for exchanging the ion exchange resin can be reduced.