Disclosed is a method for recovering a corrosion preventive agent in an electrodischarge machining liquid supply apparatus. The method includes the step of passing a first electrodischarge machining liquid having a corrosion preventive agent added there to through, an ion exchange resin column to maintain a suitable specific resistance of the first liquid, and the step of passing a second liquid free from a corrosion preventive agent through the ion exchange resin column to desorb the corrosion preventive agent adsorbed in the ion exchange resin column. The second liquid is, for example, tap water or a machining liquid having a low corrosion preventive agent concentration. The corrosion preventive agent is, for example, adenine which is a purine base.