An image reading apparatus includes: a light source emitting a plurality of colors; a light receiver including a plurality of pixels which receive lights of respective colors; an analog-to-digital converter which converts analog signals obtained by the light receiver to digital photoelectric conversion outputs, respectively; a color determining portion which receives output signals, determines whether output signals for respective colors from the same pixel are within predetermined ranges and outputs, as dropout data, pixel position information of the pixel, for those output signals that are within the predetermined ranges; an image monochromating circuit which receives output signals and obtains monochrome data; a data substituting portion which receives the monochrome data, substitutes the monochrome data corresponding to the pixel position information from the color determining portion with a level at which the pixel is determined as a white level in a binarizing process.