An image processing apparatus for converting a first image having a first range of a brightness into a second image having a second range of the brightness narrower than the first range obtains information of sizes of the first range and the second range, divides the first image based on the information such that the number of areas obtained by dividing the first image when a ratio of the second range to the first range is a first value becomes smaller than that when the ratio is smaller than the first value, sets, for each of the areas, relationship information that associates input brightness values in the first range with output brightness values in the second range, and converts brightness values of pixels included in the first image into brightness values of pixels of the second image based on the relationship information.