1. A method for applying site activity indexes by a site activity index applying system connected to a plurality of user terminals through the Internet, the method comprising: determining whether a user desires to buy a game item for use in a game provided on a web site; determining whether to use the user's site activity indexes so as to buy the game item; determining whether the user possesses the site activity indexes; and exchanging the site activity indexes with one or more game items in response to a determination that the user possesses the site activity indexes, wherein the site activity indexes comprise numerical activity indexes provided to the user according to the user's activities on the web site, and wherein the user's activities for providing the site activity indexes comprise at least one of a transaction activity on the web site, an activity for viewing advertisements in the web site, an activity for joining events in the web site, an activity for transmitting or receiving mail through the web site, and an activity for logging in to the web site.