PURPOSE: To express the outline image data prepared from a bit map, graphic data such as segmented mask data, etc., without collasing the shapes of their detail parts and to express also a curve changing largely and smoothly by a minimum number of point.

CONSTITUTION: As for outline graphic data expressed by the sets of the sequence of points of the coordinates of a two-dimensional plane, the linear component is segmented from the change of a vector composed of a few continuous points. The noise component is decided from the angle and distance formed by the vector prepared by the few continuous points of the coordinate of the sequence of points of the segmented outline and the component is eliminated. In the case of the angle changing gently and largely, the distance from an origin is always monitored. When the distance exceeds a prescribed one, the present point is preserved, a processing is performed by defining the point as a new reference point and the shape of the angle is kept. A vector to be formed by connecting points with each other by skipping one or more points for oblique line which largely become step-wise is determined. The whole changing direction is determined, is made into a straight line and the number of point is decreased.