To easily and also efficiently generate a graphic shape by judging which vertex of the graphic shape exists on which side of a control line inclined at an optional angle and moving the vertex of a graphic shape existing on either side of the control line in the orthogonal direction against the control line as much as an extending distance.

A dialog box 36 receives control data that are used as input to an extension controller from a user. The user uses a box 33 and shows whether an extension direction is positive or negative to a control line or both of them. A box 40 shows the user the direction of the control line and the direction selected in the box 33 in a diagram. A box 34, the user input an extension distance. This distance can be a constant, a variable whose value is given by the user when it is inserted or an extension consisting of 1, plural variables and (or) constants.