The present invention permits to determine an image characteristic amount for efficiently searching an image without affecting a bad influence upon searching accuracy. First image characteristic amount data is extracted. Second image characteristic amount data is extracted. Then, a distance between two image characteristic amount data is calculated. Further, the calculated distance is compared with a threshold value. If the distance is smaller than the threshold value, the image characteristic amount data having smaller number of coefficients which is advantageous because of smaller data amount is selected, and, if otherwise, the image characteristic amount data having greater number of coefficients is selected.