1. An image searching apparatus which searches for an image that is similar to a query image from among a plurality of images to be compared with, comprising: an extraction unit adapted to extract a color feature parameter and a luminance feature parameter of the query image; a first derivation unit adapted to compare the color feature parameter of the query image with the color feature parameter of each of color images to be compared with, and to derive a color similarity that denotes a degree of similarity therebetween; a second derivation unit adapted to compare the luminance feature parameter of the query image with the luminance feature parameter of each of color images and grayscale images to be compared with, and to derive a luminance similarity that denotes a degree of similarity therebetween; a transform unit adapted to transform the luminance similarity of each of the grayscale images to be compared with and the query image into an integrated similarity that is integrated in accordance with a correspondence between the color similarity and the luminance similarity of each of the color images to be compared with; and an output unit adapted to output the images to be compared in order by similarity, using the color similarity of the color images to be compared that is derived by the first derivation unit, and the integrated similarity of the grayscale images to be compared that is transformed via the transform unit.