To improve the approximation accuracy and processing speed when a tertiary Bezier curve is subjected to line segment approximation.

When data of a tertiary Bezier curve is given, it is judged whether it is such a rare exception case as the start point and the end point of the curve coincide with each other, and the curve is recursively divided (S20) when it corresponds to the case. It is judged whether a segment of the divided result is in a projecting shape or not, and the segment is recursively divided (S30) until a segment that is not in a projection shape is acquired. Next, it is decided whether the segment that is not in the projecting shape is in a crossing shape or not, and the segment is recursively divided (S40) until a segment that is not in the crossing shape is acquired. An acquired segment of a standard shape is recursively divided until the distance of a straight line that connects the middle point of two controlling points and start and end points becomes an allowable value or less, and when the distance reaches the allowable value or less, the segment is undergone segment approximation.