A method for designating a representative color using a color as information with respect to an image region, for use in an image search system, the method comprising; determining a reliability for the image region; and designating a representative color value in a color space that represents the image region, characterized in that the reliability for the image region is determined based on a mixed rate and a color similarity of the colors of the image region, wherein the mixed rate of the image region is obtained using, for each color of the image region, a ratio that said color occupies in the image region with respect to all colors of the image region and an expansion rate which represents a degree that said color is mixed with other colors in the image region, and said representative color value of the image region is determined based on a comparison of the reliability with a first threshold value, wherein i) in the case that the reliability is larger than the first threshold value as a result of said comparison, the average value of all color values in the image region is designated as the representative color of the image region, and ii) in the case that the reliability is smaller than the first threshold value as a result of said comparison, designating the representative color of the image region comprises: a) obtaining the number of pixels having a chrominance exceeding a predetermined value in the image region and the chrominance color frequency of the image region by dividing the obtained number of pixels by the number of all pixels in the image region; b) in the case that the obtained chrominance color frequency is not larger than a second threshold value, the average value of all color values in the image region is designated as the representative color of the image region, c) in the case that the obtained chrominance color frequency is larger than said second threshold value, c1) extracting a hue element from the color of the pixel with respect to all pixels in the image region and forming a hue histogram by a descending series; c2) the bin values are summed in the bin sequence until a result of the summing operation exceeds a third threshold value; and c3) when the result exceeds the third threshold value setting a color set corresponding to the summed bin, obtaining an average of the pixels in the image region corresponding to the color set, and designating the obtained average as the representative color. i) in the case that the reliability is larger than the first threshold value as a result of said comparison, the average value of all color values in the image region is designated as the representative color of the image region, and ii) in the case that the reliability is smaller than the first threshold value as a result of said comparison, designating the representative color of the image region comprises: a) obtaining the number of pixels having a chrominance exceeding a predetermined value in the image region and the chrominance color frequency of the image region by dividing the obtained number of pixels by the number of all pixels in the image region; b) in the case that the obtained chrominance color frequency is not larger than a second threshold value, the average value of all color values in the image region is designated as the representative color of the image region, c) in the case that the obtained chrominance color frequency is larger than said second threshold value, c1) extracting a hue element from the color of the pixel with respect to all pixels in the image region and forming a hue histogram by a descending series; c2) the bin values are summed in the bin sequence until a result of the summing operation exceeds a third threshold value; and c3) when the result exceeds the third threshold value setting a color set corresponding to the summed bin, obtaining an average of the pixels in the image region corresponding to the color set, and designating the obtained average as the representative color. a) obtaining the number of pixels having a chrominance exceeding a predetermined value in the image region and the chrominance color frequency of the image region by dividing the obtained number of pixels by the number of all pixels in the image region; b) in the case that the obtained chrominance color frequency is not larger than a second threshold value, the average value of all color values in the image region is designated as the representative color of the image region, c) in the case that the obtained chrominance color frequency is larger than said second threshold value, c1) extracting a hue element from the color of the pixel with respect to all pixels in the image region and forming a hue histogram by a descending series; c2) the bin values are summed in the bin sequence until a result of the summing operation exceeds a third threshold value; and c3) when the result exceeds the third threshold value setting a color set corresponding to the summed bin, obtaining an average of the pixels in the image region corresponding to the color set, and designating the obtained average as the representative color. c1) extracting a hue element from the color of the pixel with respect to all pixels in the image region and forming a hue histogram by a descending series; c2) the bin values are summed in the bin sequence until a result of the summing operation exceeds a third threshold value; and c3) when the result exceeds the third threshold value setting a color set corresponding to the summed bin, obtaining an average of the pixels in the image region corresponding to the color set, and designating the obtained average as the representative color.