A Bezier curve constituting outline font data represents a curve by four coordinate values of a start point, an end point, and two control points. The curve is represented by two coordinates of the start point and the end point in collinear approximation, and by three coordinates of the start point, an intersecting point of two lines, and the end point in bi-linear approximation. In curve pattern approximation, when 32 curve patterns are prepared, the curve is represented by 5-bit data of two coordinates of the start point and the end point, and a pattern number. By effectively employing these three kinds of processes, it is possible to represent a Bezier curve constituting the outline font data with a reduced data volume.