1. An image processing apparatus, comprising: a storage unit configured to store an index value for each of a plurality of mutually differing degrees of variation; a first obtainment unit configured to obtain a degree of variation of signal values included in a region of interest of a predetermined size including a pixel of interest included in image data; a second obtainment unit configured to obtain a value indicating a brightness of the region of interest; a determination unit configured to determine an amount of correction of a value of the pixel of interest using the value obtained by the second obtainment unit and an index value corresponding to the degree of variation obtained by the first obtainment unit that is stored by the storage unit; and a correction unit configured to correct the value of the pixel of interest using the amount of correction determined by the determination unit.