1. An image processing apparatus for executing tone correction of luminance values in an image, comprising: a generation unit configured to generate, for the image, a luminance histogram in which frequencies indicating numbers of pixels for respective luminance values are arranged in an order of luminance values from a first side to a second side; a dividing unit configured to generate a plurality of partial histograms by dividing the luminance histogram at downward convex parts in a frequency distribution of the luminance histogram; a calculation unit configured to select one of the plurality of partial histograms as a partial histogram of interest, to calculate, as a drop candidate, a difference between a maximum frequency in one partial histogram located on the first side of the partial histogram of interest, and a minimum frequency within a range from a luminance level corresponding to the maximum frequency to a luminance level corresponding to a maximum frequency in the partial histogram of interest, and to determine, as a drop of the partial histogram of interest, a maximum one of the drop candidates calculated for all the partial histograms located on the first side of the partial histogram of interest; a setting unit configured to set, for the partial histogram of interest, a smaller weight with increasing drop; and a correction unit configured to execute the tone correction using a luminance histogram obtained by applying weights set by said setting unit respectively to the plurality of partial histograms.