Disclosed is a resolution conversion method, which makes it possible not only to acquire a diagonal edge that is smoothed without causing any unevenness all over the image, but also to stably reproduce a thin line having a thickness corresponding to its original thickness, irrespective of an integer multiple or a non-integer multiple of the magnification factor to be employed for enlarging an original image, when the high-resolution conversion processing is applied to a binary image represented in the dot-matrix format. An operation for integrating the interpolation value with respect to the area of the input pixel is implemented after the output pixel area is shifted in a horizontal direction and/or a vertical direction so as to make the output pixel area depart from a center of the interpolation area, when the output pixel area is included in a single interpolation area in the horizontal direction and/or a vertical direction.