It is an object of the present invention to output a clear, expanded image wherein step-shapes or chain-shapes of oblique lines are reduced, distortion is eliminated and no constituent lines of fonts or graphics are missing. An image transform method, for transforming original input image data into image data expanded by a ratio represented by a rational number or an integer, comprises: a vertical and horizontal interpolation unit, for reducing correlation in the vertical and horizontal directions of interpolated image data linearly expanded from an original image data input unit, and for generating first expanded image data; an oblique interpolation unit, for performing linear interpolation, based on correlation with a target pixel constituting the original image data and neighboring pixels arranged in oblique directions, using the neighboring pixels to generate second expanded image data; and a final result generator for employing the first expanded image data and the second expanded image data to generate a final image.