An image reading apparatus includes a reference pattern having slanting lines, which is provided outside a reading range and read by image sensors arranged at separate positions in a sub-scanning direction. A reference-position determining unit detects one of the slanting lines in the reference pattern based on image data output from one of the image sensors so that a position of the image data when one of the slanting lines is detected is determined as a reference position. First and second delay units have line memories which store image data read out from an original image having lines, the delay units delaying outputting of the image data from the line memories line by line. First and second determining units determine image data having color values at imaginary points. An error measurement unit selects one of the imaginary points whose image data has a minimum difference between the color values of the image data. First and second line correcting units output image data having the color values at a corrected position of each picture element by using an interpolation function, the interpolation function having a set of correction factors determined based on a distance between the selected imaginary point and the reference position.