1. An image search server which searches for a print request having an image similar to a query image transmitted from a search client on a network, comprising: a first reception unit configured to receive a print request, from a device having a printing function on the network, which contains image data to be printed by the device and additional information containing at least information indicating a print request source of the image data; a registration unit configured to register the image data contained in the print request and which has been printed by the device, feature amount information extracted from the received image data, and the additional information in a storage unit; a second reception unit configured to receive a query image from a search client on the network; a search unit configured to extract feature amount information from the query image and search for an image from the registered image data which as been printed by the device based on the extracted feature amount information of the query image; a classifying unit configured to classify images found by said search unit, based on print request sources of the additional information of each of the found images; and a search result transmitting unit configured to transmit the classified images corresponding with the query image and the additional information corresponding with the classified images as a search result to the search client so that the search client can detect a device as the print request source which sent a print request for printing an image similar to the query image.