1. An image matching apparatus comprising: a computer-readable memory constructed to store computer-executable process steps; and a processor constructed to execute the computer-executable process steps stored in the memory; wherein the process steps stored in the memory cause the processor to perform: a detector step of detecting, from an input image containing an object to be matched, a region where the object exists; an estimation step of estimating a parameter regarding a luminance value based on an image of the region; a correction step of correcting the luminance value of the input image based on the parameter; and a matching step of matching the corrected input image containing the object to be matched with an image containing a matching object, wherein, said correction step corrects the luminance value of the image by: changing a luminance value smaller than a first threshold value to a minimum luminance value, changing a luminance value larger than a second threshold value to a maximum luminance value, and linearly interpolating a luminance value located between the first threshold value and the second threshold value.