PURPOSE: To improve the machining precision by determining the working area of a workpiece on the basis of voltage generating time, determining the working time on the basis of this working area and the working time per unit working area to control the machining of the workpiece.

CONSTITUTION: When an electric discharge machining is performed under working conditions of n-stages, the working conditions are switched by a control device 10. In this case, at the n-1 stage working, a working area S is determined by a working area arithmetic device 25 on the basis of the n-1 stage working space distance preset in the working area arithmetic device 25 and the voltage build up time from a measuring circuit 24. On the basis of the working time (t) per unit working area from the n-1 stage working surface roughness to the n-stage working surface roughness, which is preset in a working time input device 19, and the calculated working area S, the ut-stage working time Tn (Tn=t.S) is arithmetically determined by a working time arithmetic device 20. Thereafter, this calculation result Tn is set in a working time setting device 12 as the n-stage working time.