This paper describes the process reaction force which is generated in EDM by the explosive expansion of a bubble due to evaporation and dissociation of the dielectric liquid. The reaction force influences on the machining accuracy because the position of the tool electrode is displaced under the reaction force. Thus in this research, the reaction force is calculated by integration not only the pressure inside the bubble but also the pressure in the dielectric liquid distributed over the working surface. We compared the calculated results with the reaction force measured in a single discharge and obtained the relationship between the reaction force and the machining conditions. Then we also measured the reaction force in a series of pulse discharges and obtained the following results: (1) The pressure distribution in the dielectric liquid in the working gap accounts for the most part of the reaction force. (2) Hence the reaction force is almost proportional to the area of the working surface. (3) The reaction force in a series of pulse discharges decreases with the lapse of time as the working gap is filled with the generated bubbles in place of the dielectric liquid.