1. An image search server which searches for an image similar to a query image transmitted from a search client on a network, comprising: first reception means for receiving image data to be printed and additional information containing at least information indicating a print request source of the image data from a device having a printing function on the network; registration means for registering received image data, feature amount information extracted from the image data, and additional information in storage means; second reception means for receiving an image search request from a search client on the network; search means for, when said second reception means receives a search request, extracting feature amount information from a query image contained in the search request and searching for an image similar to the query image by using the extracted feature amount information; classifying means for classifying the found image on the basis of additional information of the found image; and search result transmitting means for transmitting the classification result as a search result to the search client.