PURPOSE: To eliminate the round part formed in a concaved part and improve the accuracy in a rectilinear part by allowing a relief part to be formed simply at the part where correction is necessary in a concaved part, in correcting the shape of a corner part in wire-cut electric discharge machining.

CONSTITUTION: In machining a relief part in a concaved part, with the radius of the relief part which is equal to an offset value, an instruction M1(x1, y1) is given at A point, and the next instruction is M2(x2, y2). In other words, the instruction M1 for machining in rectilinearly from the position of A point to the position x1 on X axis and y1 on Y axis is given, and next instruction M2 is the order for machining continuously in rectilinearly up to x2 on X axis and y2 on Y axis. Therefore, it is judged that the surface to be worked according to these instructions M1 and M2 has the corner in concaved form or convex form, and if it is a concaved part, the above-described relief part is formed. Thus, the round part formed in the concaved part is eliminated, and the accuracy in the rectilinear part is improved.