An apparatus comprises: a determination unit that determines whether target pixel data of a reference member is a singularity pixel; a counter that counts the number of pixel data that is not determined as the singularity pixel, for each of main-scanning positions; a summation unit that cumulatively sums the target pixel data not determined as the singularity pixel, for each of the main-scanning positions; and a calculation unit that, for each of the main-scanning positions, causes the summation unit to cumulatively sum the target pixel data not determined as the singularity pixel if a count value corresponding to a main-scanning position of the target pixel data is smaller than a predetermined number, and calculates shading correction data using data that the predetermined number of pixel data not determined as the singularity pixel are cumulatively summed.