When tetrahedral interpolation is used in correction of color reproduction upon color adjustment, image disturbances such as pseudo edges and the like may occur since the surfaces of each tetrahedron satisfy the boundary condition. Hence, a pixel position of digital image data is calculated, RGB values at that pixel position are acquired, and the acquired RGB values are displayed on RGB value display area. L*a*b* values are calculated based on basis functions and coefficient values of a color calculation B-spline solid, and the calculated L*a*b* values are displayed on an L*a*b* value display area.