1. An apparatus for enhancing the quality of reproduced images, comprising: a vertical area existence determination unit which determines whether or not an edge included in an input pixel 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; and 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.