1. An image search system for determining a similarity of an image whose feature are represented by either one of image features amounts, a color distribution features or a frequency distribution features, to search for a similar image, comprising: means for converting, with respect to an image set to be a target whose kind of image features amount is to be changed among respective images to be searched and an inquiry image, a kind of image features amount of the image in question to make kinds of image features amounts of each said image to be searched and said inquiry image coincident with each other; and means for comparing the image features amount of said inquiry image with the image features amount of each said image to be searched based on said converted image features amount and determining a similarity of each image to search for a similar image.