1. A network system in which image data items are uploaded from a plurality of user terminals to a server and images are opened to public among the users, the network system including at least one central processing unit, the system comprising: an uploaded image storage unit configured to store image data items uploaded from the terminals; a work storage unit configured to store a plurality of image data items stored in the uploaded image storage unit as works for publication; a classification unit configured to classify the plurality of works stored in the work storage unit into a plurality of categories; and a category control unit configured to (i) divide the works classified into the categories into a first group of works with each of which a number of times of browse requests for a work from the terminals is greater than or equal to a predetermined number, and a second group of works other than the works in the first group, and classify one of the first and second groups of the divided works as another category different from the categories, and (ii) execute one of category unification, category division, and category deletion in accordance with a number of works included in each category, wherein category unification occurs by unifying one category with another category, category division occurs by dividing a category out from another category, and category deletion occurs by deleting a category.