An electrical discharge machine is disclosed having a concentration detection function for a rust inhibitor containing an organic compound uses coloring of a metal complex produced through the reaction of the rust inhibitor with a color reagent to enable a detector to detect the change of characteristics involved in the change of the color of the metal complex. A predetermined amount of working fluid is sampled in a sampling cell at regular time intervals, a predetermined amount of color reagent is added to the working fluid, and the change of the color of the working fluid is detected by the detector.