1. An apparatus for enhancing the quality of a reproduced image comprised of a plurality of pixels having original pixel values corresponding to the image, comprising: a vertical area existence determination unit which determines whether or not an edge included in an input pixel selected from the plurality of pixels belongs to a vertical area; a slant line possibility determination unit which determines whether or not there is a possibility of the edge forming a slant line when the edge is determined to not belong to the vertical area; a direction determination unit which determines a direction of the slant line when the edge is determined to possibly form the slant line; an interpolation unit which calculates an interpolated pixel value for the input pixel based on the determination results provided by the vertical area existence determination unit, the slant line possibility determination unit, and the direction determination unit; and an adjusted pixel value calculation unit which compares the interpolated pixel value with an original input pixel value and adjusts the interpolated pixel value by designating one of the original input pixel value or the interpolated pixel as an output pixel value based on the comparison result, wherein the adjusted pixel value calculation unit designates the original input pixel value as an output pixel value when a difference between the interpolated pixel value and the original input pixel value is not greater than a predetermined threshold value, and designates the interpolated pixel value as the output pixel value when the difference between the interpolated pixel value and the original input pixel value is greater than the predetermined threshold value.