An image analyzing section acquires information on a type of a subject as an image attribute using an image analysis technique, and if the subject is a human, information on clothes of the human as the image attribute. In an additional information DB, there are provided plural pieces of candidate additional information which possibly become additional information, plural reference attributes for each piece of the candidate additional information, and individual compatibility degrees for the respective reference attributes. For every piece of the candidate additional information, a sum of the individual compatibility degrees of the reference attributes which are correlated with the image attributes acquired from the image are calculated as a total compatibility degree. The candidate additional information having the total compatibility degree equal to or higher than a certain value is selected as additional information. A tag having this additional information is attached to an image file.