PURPOSE: To calculate a normal vector at a high speed.

CONSTITUTION: In the graphic processing method for generating an image, based on apex data of a polygonal graphic, based on graphic data containing coordinates of two points and a normalized normal vector, a coordinate calculating means 5 for deriving a coordinate of a point of a segment for connecting two points, and a normal vector calculating means 2 for deriving a normal vector of an interpolated point are provided, and the normal vector calculating means 2 is constituted so that an angle made by the normal vectors of two points is calculated, and also, the normalized normal vector of the interpolated point is calculated, based on the normal vectors of two points and a divided angle of its angle.